back in 2008 december , i had almost the same rig as u!! don't know the mobo, but the processor was a core 2 duo e7400 O.C to 3.0 GHz , and HD 4870,l 3 gb ram ,, and i had the same problem , gpu temps were unbearable!!! over 100c when gaming!!!! and the problem was with the gpu fan , it was not working properly , so i replaced it with a new one!! the problem was solved , ,, but your issue seems a bit different,.,.,.,..,! u shud still check the fans!