OK : I'll be honest with you :
I'm using Vista x64 Ultimate...
I ONLY play COD4 online, so that's the only game I can give you stats from.
In XP I get a constant 150+ fps
In Vista X64 Ultimate I get 100fps constant. (1680x1050, full graphics)
So : Running XP seems to be better when it comes to games, BUT, if you're not VERY serious about being able to use the 250fps exploit in COD4, you've got NO trouble in any games with a quad core and an 8800GT.