You've got 8GB of RAM. I honestly don't think that you would have a memory problem. Vista has the added bonus of using DX10 and since you've got the graphics card, well I reckon you should stay with Vista.
For now I'd say, without a doubt go for XP, and wait for Windows 7 by the end of the year! See PCAdvisor, But then again, maybe you posted this tread just to brag about your specs... you've got reason to, I'm very jealous!
Personally I'd go for Vista 64 bit, I have no problems and generally its actually not as bad as people claim if your PC is good enought and all I ever hear about Windows XP 64 bit is the lack of driver support...
If you plan on making any major changes, make a mirror backup of the drive, then if anythig happens and for whatever lunatic crazy reason your not happy with a faster better operating system, you can go back to vista, A computer should run your OS fast, you shouldnt need a fast computer to run your os...
I have downloaded a free application and that mirrors so going to give that a try and if all else fails hopefully I can revert back :P (And yes it was Legit free )