I had same cards the 8800Gtx it was indeed heat related problems leading to artifacts and crashes, I binned them and moved up to 9800GX2 cards,...
8800 GTX is known to overheat easly, id try temp monitor, keep it wrighting temp logs while your playin the game, if the temps are shooting up b4...
Speedstep I would be more inclined to leave speedstep off its a power Saver function, ( Runs your CPU u faster on load, and less voltage off...
Separate names with a comma.