1) Heat (Your card could be overheating causing the glitches)
2) Internet Bandwidth (It looks like you are playing online and if you have issues with your badnwidth (pin) you can get glitches.)
Thanks for the reply.
I've tried 2 (one from ASUS one from XFX) 7600 GS and both give me the same problem. The problem shows up whenever the game is played, but I think the amount of glitches on screen might have increased the longer I played but I'm not sure. Also, this problem only occurs in 3D games, I have no trouble using the card to watch 720p videos. Is it still likely to be a heat issue? Would dusting the innards of the case help? (It's a bit dusty in there)
I'm sure it's not a bandwidth issue, my ping is below 150. I've also played L4D2 on other computers in the house without any connection problem.