The most demanding games out there that i can think of would be F.E.A.R, Battlefield 2, and Call of Duty 2 is also pretty demanding. With a 6800GS, you should be able to max out those games, or come very close to it.
AMD 64 3200+ Newcastle@ 2.2Ghz
1GB (1x1GB) OCZ Performance Series
ATI X700 Pro 256MB OC\'d 475/525
OCZ Modstream 520W
Samsung 914v 19\" 12ms LCD
SB X-Fi w/ Logitech Z 5300e