look tward the bottom of this thread:
also waht is the specs of yoru PowersupplY? (the 12v amperage is more important then the wattage ratting). also witch brand is it.
though your porb sounds similar to software/driver issue. i had a similar issue as that one time.... A clean reformat of Windows is a shur fire way to fix any such issues.
... my 6800GS can run any game iv tried so far(HL2/Farcry/Doom3/FEAR) at 1200x1024 res /High settings/ 2xAA/ 16xAF (at least, and with steady smooth FPS/ average at least 50fps or higher).
though since i have a 19" crt, i run them at 1600x res, 0xAA, 16xAF. with still similar steady FPS.
the 7600GT is better, once you fix your problem, you should beable to run any such games near Max settings without issue.