Almost any computer you can build today will run all current games at or near max settings because the games are dummed down ebough to run on 5 year old consoles. You won't have to worry about your next computer until game designers begin designing games for the next generation of consoles.
last post is oooo so true! (ram1009) I had 4850's in CF until recently and they played everything I own (crysis, fallout, far cry 2, stalker cop) on max without AA at 1920 x 1080. Bought a 6870 but it was stupid as the only game I really could use it for would be metro 2033, but I'm not interested in it. A good rig from a year or 2 ago (ATI 4800 series in CF or 5800s will still play everything there is at the moment, for the reasons mentioned by ram:
All big games are made for PC and console, so because the designers and artist can't use large textures 8k and above on console, the dumb it down rather than see their work go to waste on a console.... what a pity!