Depends on your setup and requirements. If it were my decision, (and it may be soon~ish, as I'm itching for a project and my 8800GTX is getting old...), then I'd check memory requirements. Why? Because that's a Honkin' Big Card!
The system is going to need 500~750 Mb worth of address space to run (Bios, communications, etc), and a 4870x2 adds another full GB to the mix. So if you plan on sticking with 2GB of RAM and a single 4870x2, then you're OK with whatever OS you feel like using, since the computer should consume 3.5 to 3.75GB worth of address space. At this level, you are perfectly GTG.
There is a potential issue if you want more RAM, though - a 32 bit OS can only address 4GB worth of memory mapped I/O. So if you installed 4GB of RAM to the above system, then you'd only pick up another 250~500MB worth of usable RAM. Now you have a problem - The memory you bought can't actually be used because the operating system has run out of address space.
And if you're going to CrossfireX with the thing, that's 2GB of memory mapped I/O for graphics. Add in another 500~750 MB for the system, and you only have 1.25GB of address space for RAM. So at this point, you are basically forced to use a 64 bit operating system if you want enough RAM for new games to perform well.
So - If you want the first setup, then you can use anything you like. Vista's bugs have been worked out for the most part and the performance gap that existed at release has been closed now that makers have figured out how to write good drivers. References:
But a lot of people don't like Vista, or don't want to change, or whatever - and 32 bit XP is still an excellent choice. If you like it, then use it please.
But if you're after the latter setup, I would opine that Vista 64 is the way to go. Why? To use the Windows logo on new products, it *must* have drivers for Vista 64 as well as 32. So when you buy new stuff, you will be able to get drivers. And YES, your 32 bit programs will run, (and on a binary level - Natively). XP64 is still good, but development for this OS has basically ceased and it has somewhat limited support to begin with - Though many XP64 users mirror the experiences of Vista 64 users and report that incompatibility rumors are overblown and over hyped.
Sorry if this comes off like an ad, or whatever - I just wanted to make the point that IF you put yourself in the position to need the additional address space, then Vista 64 is the way to go. If not - Please use whatever 32 bit OS you choose - For many/most this would be XP.
Its all about relevance. If you are getting 80fps in XP, so you may only get 70-75 in Vista, so what? If you are only getting 30 in XP, you may only get 28 in Vista. Vista is a more demanding OS. But if your hardware is current, good dual core CPU, 3 gig or more of memory, your games will run just fine and the difference won't be noticable.
For instance, if you have a AMD 3200+ single core CPU, 1 gig of memory, and a 3850 GPU, you are probably better not going with Vista for gaming, as your hardware is going to struggle with the latest games with eye candy turned up very far anyway.
If you have a fast dual/quad core, 2-4 gig of memory, and a 4870X2, Vista is not going to bother you at all, and you can run with very high settings and everything will look pretty.
Vista is decent OS, I think it is far superior to XP. No reason really not to make the move to Vista unless your PC is just not up date with modern/Vista compatable hardware.