8800GT is a great card and it will run very nicely under WinXP, but you can't go wrong dual booting it with Vista64, this way if there are any important apps that simply won't work on Vista64 you can go back to XP, or you can just use Vista64 for DX10 games.
I know Crytek claim that the 64bit version runs the best, but I have no idea where I read it (maybe gamespot?) so I can't link it
XP/vista32 can only see 3GB of Ram but Vista64 can see 4GB and above, even though few programs require more then 2GB, it's only a matter of time till 3GB is no longer enough.
Even though you'll lose nothing by dual booting it I find it hard to recommend Vista as your main Gaming OS, as games do tend to run slower under it- how much slower depends on the game, hopefully as new drivers for graphic cards and Patches/service packs for Vista get released this situation will improve.
"..just keep on trying 'till you run out of cake"
