i find games and vista dont go together at all at the moment, i now have 2 set ups 1 with vista and one with xp pro and this is the one i use for games, even lotro and supreme commander which are very recent games perform very poorly with vista, on a very high spec pc.
however if you go with vista, as im sure we shall all have to do, maybe look at a dx 10 ready card (a g80 for gaming) or see what ati release, and if you're looking to clock it, as the cpu will possible not supply the gpu fast enough i would get faster ram (800mhz)
obviously this would mean a bigger psu.......
it never ends! just depends how far you want to go. with xp that would be a great setup, not too sure with vista though.