Your temps are normal for a 6600GT. I play for hours on end at 70° C, with my eVGA 6600GT, without artifacts or any other problems. I have noticed problems like this with some driver versions, but my current 81.98 drivers are flawless for me (your mileage may vary)
You may want to try the newest 84.43 beta drivers from www.nzone.com, though I don't know if that's the problem or not. How are your other temperatures, such as CPU and PSU temps? One time when I was having artifact problems, it was my system memory (specifically, my DIMM slots had dust in them, and needed cleaned out) rather than gfx card causing my problems, but I had occasional bluescreens as well, when this happened.
Oh, and I agree totally with the others; 6600 isn't up to the beating that Oblivion and FEAR will put them through at higher graphics settings. Oblivion will strain a 6800GT, let alone a poor low-end card like a 6600GT (though if you're like me, it was all you could afford, and is pretty good, for the amount of money one has spent...)