Ok so I just bought a brand new msi gtx 660 twin frozer video card to replace my old asus 6850. After uninstalling my old amd drivers I went ahead and switched out both video cards and downloaded the new 310.9 nvidia drivers for my card. Now what I've noticed from playing a few games is that depending on which settings I have enabled my computer will just shut off and not only that I actually have to flip the switch on and off on my psu before I can turn it back on. The Games I tested were Batman Arkham city which plays fine until I turn on the direct x 11 feature then once I start the game after the menu screen the computer shuts off and Saints row 3 does the same with all the settings on high. Borderlands 2 plays perfectly with everything on max and so does Left for dead 2 and Dishonored and mind you I only played these 3 games for about 20 minutes each but my computer didn't shut off.
I'm not entirely sure whats going on here exactly but I would think my problem is leading more towards the psu rather than the graphics card? any help would be appreciated.
Another thing I noticed is once I turn the psu power switch off after a shut off I hear some kind of power surge sound, not sure if that helps at all.
Not really experienced in this area but try remove all the drivers again, and re install and see how it goes, have you tried an alternate psu? a surge protector if the other games run fine then it shouldn't be card related. Is your psu powerful enough to run the 660? the 660 requires more power. it seems to happen once load gets too high, psu model and other components list would help have you tried putting your old card back in and running it?
it looks good, the efficiency isn't that high check out some reviews or ask for help picking a new one in the forums, spending extra for one that has high efficiency and will last long will save more money in the long run.