well thats not really something we could say, unless we knew your temps. i dont really know how much heat the 6600 series in general will take, but i dont imagine its much different from the 6800 series. and the only thing that could possibly run hotter than one of those is a dual core prescott
if you start seeing artifacts, then obviously, back it off.
Dude, you can run those cards up to 90c load without a problem. Just try to keep it in the 80's and you'll be fine. The 6600GT has some crazy thermal threashold, 140c I think.