Can't get rid of artifacts

I OC'd my 7900 GS up a bit and it wasn't causing any problems until I tried to play Gunz then it started showing artifacts when I tried to play so I tried CSS as well and its also getting them..all my other games are fine though.

So I put it down to factory settings, turned it off for 20 minutes then just turned it back on and I STILL have artifacts on those two games....any help?

Hmmmm, artefacts mean that your card is ****ed (irreverasbly) or that you have outdated drivers/incompatible software.
What OS are you running on?
Use ATITools to check if your card shows any real artefacts (it doesn't work too good with vista but I can tell you how to get it to run.)