Help diagnosing video artifacts and crashes

So I've been getting a lot of artifacts and crashes when playing games.

Here are my specs:
evga 680i MB
PNY 8800gt
Core 2 Duo 6600
2GB corsair DDR2 PC26400 (800 MHz)
Raptor 10,000 RPM 74GB / WD 7200 RPM 250 GB
650W PSU

At first I thought it was my the 8800gt, but I replaced it with my old 7600gt and even though it would not get all the artifacts I still get crashes some time, although not as often. Because of this I'm starting to think that it may not be my video card. I have tried using the other PCI Express slot on the mother board, but still same results with both video cards. I'm leaning toward a MB or PSU malfunction but I'm not sure, and wanted to get some input before I go pick up some new hardware. I have tried many different settings on my MB's bios including the default settings.

I would appreciate any ideas of what the problem could be and/or suggestions for tests to determine the problem. Also if anyone knows any settings I could adjust in the MB's BIOS that could stabilize my system.

Thanks.