If you have the machine to support it, vista is actually going to perform the same if not better in most games. Vista has much better usage of memory, and is definately faster than xp when it comes to loading stuff. The only reason i keep XP around is that i get a slightly higher (around 5%
score on 3dmark06. But if you already have a copy of xp laying around that you can use, yes, buy the 4870 instead.
My point is that if its a "i have to buy an OS" question, there is 0 reason to BUY xp when you can get vista for the same price or even less in some instances.