Magnificence is the big spender's virtue. It is one of the Aristotelian eleven (sometimes twelve, depending on precisely how they are listed) virtues that structure the discussion of the Nicomachean Ethics (IV.ii). Two of these virtues are devoted to doing things on a grand scale, megalopsychia or magnanimity, and megaloprepeia, or magnificence. My understanding is that outside of Aristotle and lines of discussion influenced by him, the two words are usually synonyms in Greek, and it is an interesting feature of their history that magnificence has a long history of being conflated with magnanimity even in other languages. Perhaps the most interesting example of this is Spencer's Letter to Raleigh at the beginning of the Faerie Queene, in which he attributes the Aristotelian virtue of Magnificence to Prince Arthur when it is in fact clear that he must mean Magnanimity, although it's difficult to know what to make of it, since it has been noted at least since C. S. Lewis that most of what Spencer says in the Letter is wrong, including several times when he is summarizing his own poem. Spencer is not the only one to do it, however, and it is an interesting question why we tend to mingle the two together.
You can think of the Aristotelian account of magnificence as locating it at the corner of a square. There is a pair of virtues concerned with the extraordinary, magnanimity and magnificence. This pair is paired with another pair, honorable ambition and liberality (or generosity), which are concerned with the ordinary. Magnanimity is to magnificence as honorable ambition is to liberality; from which it equally follows that magnificence is to liberality as magnanimity is to honorable ambition. The magnificent person spends big on important matters; his expenditures are "vast and appropriate" in order to achieve a vast and appropriate result. It is a mean between vices that we could translate as meanness (its vice of defect) and garishness or wastefulness (its vice of excess). Since virtue grants facility in the action, the magnificent person is the sort of person who does not count the cost when the most important things are on the line. Aristotle says that generosity or liberality is a prerequisite of magnificence; the magnificence person, however, in being generous achieves excellent results on a large scale, in a way that benefits everyone. That seems clear enough, but there are a number of intriguing problems with regard to magnificence.
One is that magnificence is in the odd situation of looking like it is a virtue exclusive to the rich. Indeed, it is very difficult to determine from Aristotle exactly how much magnificence depends on being rich. He himself recognizes that it requires great resources (not necessarily money, but you need something to draw on in order to produce the great results), and even goes so far as to say the magnificent person has an excellently furnished house. But he also notes that the magnificent person will achieve the great result even at the same expense as the merely generous. And Aristotle does say that someone can exhibit magnificence in one-shot productions -- weddings are the obvious case. But even in ancient Athens, even poor parents spent as lavishly as they could manage on weddings; almost certainly far more than they do today. And you only have to go somewhere in the world where weddings retain their ancient importance to discover that even poor parents will put together large-scale multi-day wedding celebrations that invite everyone, drawing as much as they can not just on what money they have but also on their reputations with merchants, on favors owed to them by family and friends, and so forth. This is very much the sort of thing Aristotle means. Aristotle also notes that the greatness of the result is not identifiable in terms of cost; his example is that of a wonderful ball, a mere toy that could hardly cost a great amount, but which can be a magnificent gift for a boy. That is expenditure is most magnificent which involves great expense for a great object; but Aristotle's magnificent man quite clearly can be magnificent in small matters, just by being tasteful and not skimping. And there is a considerable amount of restraint in Aristotle's virtue of magnificence, because garishness is so easy to slip into. Someone who spends to feast his dining-club (an important Greek institution) as if he were hosting a wedding (which is a level of feasting far beyond the good-meal-with-flute-girls that dining-clubs typically did) is garish, vulgar, not magnificent. The magnificent person might fund the chorus in a comedy, but the garish one would do so by bringing them out in tastelessly expensive purple. And Aristotle quite explicitly says elsewhere that the practice of virtue does not require great wealth. So how much does this virtue depend on resources?
A second problem is its relation to liberality. To distinguish out the virtue of magnificence, we have to do two things: keep it from collapsing into liberality, and keep it a virtue. But it's difficult to see how this can be done: magnificence presupposes liberality, but liberality does not guarantee magnificence. Liberality already covers all monetary matters, so magnificence seems like it's just a particular form of liberality. On the other hand, Aristotle quite definitely wants to distinguish them; as far as he is concerned, they are not the same virtue at all. And given the square mentioned above, collapsing magnificence into liberality would strongly suggest that we should also collapse magnanimity into honorable ambition; but honorable ambition and magnanimity are not so hard to distinguish, and Aristotle certainly would resist collapsing them into each other, as well.
It is difficult to know how to address these problems. I would suggest that one avenue of approach, however, is to recognize that Aristotle's eleven virtues are all civic virtues, and play a role in the proper functioning of a city united by friendship in pursuit of human happiness. It is in fact the whole object of the Nicomachean Ethics to lay this proper functioning out. So we should ask ourselves what function magnificence has in the life of the city. And when we do, I think we get a clearer sense both of how it differs from liberality and of its relation to resources. Liberality covers monetary matters generally, and is the virtue that concerns how you handle money; but magnificence is really about aiming for results that greatly benefit others. Perhaps the closest translation into contemporary English is 'philanthropy'. The magnificent person spends great money on great buildings (houses, temples) that improve the city and on great events (weddings, religious festivals) that are good for the community. Even the small or private expenditures of the magnificent have this city-building tinge. Magnificence is the virtue concerned with creating the common material of civilization itself. Seen in this light, it is clear that it has to be inextricably bound up with wealth, but also cannot simply be a virtue of the rich, because nothing prevents the poor from doing the same, even if lack of immediate resources means that they have to be more resourceful themselves. The poor may not generally be able to fund temples, but the poor can still work to give magnificent gifts of the sort that circumstances put in their reach (think of Aristotle's explicit example of the inexpensive but magnificent ball appropriate for a child).
A second way to approach the matter, one that also has its advantages and that I think can be made to fit with this suggestion is that of Thomas Aquinas. Being Aristotelian, Thomas can't ignore an important Aristotelian virtue like magnificence. But, like magnanimity, it's not an obviously Christian virtue. In the case of magnanimity, the big problem is how it could possibly be reconciled with Christian humility, but magnificence is even more of a problem, since it seems to clash with the entire ascetic tradition, and it is flatly against the entire Christian tradition to claim that there are virtues only the rich can have. If you tone it down, on the other hand, the problem of its relation to liberality becomes even more acute, since it's already difficult to distinguish on any other ground than big spending. Aquinas's handling of this problem is quite ingenious. Some people have not found it entirely convincing, but there's no question that as far as it goes it's a clever handling of the problem.
Without eliminating their connections, Aquinas splits magnificence and liberality up, and does so quite cleanly. On Aquinas's account, all the virtues can be organized with respect to the major virtues -- either they just are those major virtues, or they are components of those major virtues (what he calls quasi-integral parts), or they are some specific kind of the major virtues (what he calls subjective parts), or they are more loosely related satellite virtues of the major virtues (what he calls potential parts). Aquinas's liberality and magnificence are both potential parts, satellite virtues, but however similar they may be on the surface, they orbit different stars. Liberality is a potential part of justice: it is concerned with rendering in some way what is due in some way. Magnificence, on the other hand is a potential part of fortitude: it is concerned with enduring and overcoming in some way what impedes goodness in some way. This provides another means of distinguishing the two virtues. At the same time, it blocks the idea that magnificence can be a virtue of the rich in any straightforward sense. Fortitude and all its potential parts are about enduring bad things and achieving difficult good; spending a lot of money that find easy to spend, or giving away a lot of money that you find easy to give away, cannot possibly be fortitude, in however broad a sense. To be magnificent in Aquinas's sense requires pushing yourself to the point of reasonable sacrifice to create a great work. Indeed, Aquinas explicitly states that the primary difference between fortitude in the strict sense and magnificence is that the difficulty of fortitude, the cardinal virtue, is derived from the danger to one's very persons; but magnificence, as a potential part of fortitude, gets its difficulty from the danger that one might be dispossessed of one's property. You can only develop magnificence by doing great things at some risk of abject poverty. Aquinas also insists on the relativity of greatness, which I have already noted is suggested in Aristotle himself: the poor may not be able to accomplish great things absolutely, but they may still accomplish great things of a particular kind. Even if that weren't so, Aquinas says, virtue is a matter of inward act rather than outward result: it is the aiming at the appropriately magnificent result that matters.
This still leaves the question of how magnificence relates to liberality, since it clearly still does. Aquinas gives several indications of how the two are related. First, Aquinas explicitly affirms that every liberal or generous person has the virtue of magnificence either strictly speaking or in the sense that all the lack for it is the opportunity to act; magnificence presupposes liberality, but liberality disposes immediately to magnificence. Second, liberality or generosity is concerned with moderation in our general love of money, whereas magnificence is concerned with the specific difficulties of spending to produce some great work. Third, liberality covers gifts in the proper sense, since we give presents out of generosity; but while magnificence also gives gifts, even the gift-giving of magnificence is primarily a matter of spending money for a great work. For instance, insofar as we are generous, we give gifts precisely as gifts, but insofar as we are magnificent, we give gifts as a form of spending to achieve something, Aquinas's examples being those of giving gifts to honor someone or giving gifts to benefit society. Fourth, liberality is more concerned with the concupiscible, our drive to get good things, and magnificence is more concerned with the irascible, our drive to avoid bad things.