I usually use Crysis, 3DMark06 and Audiosurf. As crazy as it seems, I often was perfectly stable in Crysis with my 8800GTX, but in Audiosurf I'd get these weird artifacts branching out of the center. It is generally a good idea to run a few different stresstests and games as sometimes your overclock will be perfectly stable in one and crash five seconds into another.
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Project: Black and White i7 920 D0 | 3 x 2GB DDR3 | EVGA X58 SLI LE
XFX 4890 | Corsair HX750 | Corsair Obsidian