Just did a little test (Crysis benchmark) on my EVGA 8800GT SC.
Up to 1000Mhz mem, everything is fine, no artifacts whatsoever.
Once above 1000Mhz (1003Mhz more precisely), I get instant artifacts and my system freezes.

I'm gonna let the mem run at it specs (950Mhz) and only push core and shaders.
Card is watercooled with EK block.