I had that problem once, not with the GPU, but with the CPU overheating.
I downloaded SpeedFan
http://www.almico.com/speedfan.php
Once I had watched it for a while to an idea of fan speeds and temps, then I start playing some of the more high end(powerwise) games. I noticed that the Temp on the CPU really climbed.
Turned out to be the cooling fan on the CPU had taken a dump. It wasn't dead but the RPMs were low (1000 instead of 5000). Replaced the fan and all was fine.