Where did I mention I "ONLY" play games in this machine??? "its the game and not the system as everything else runs super fast." did you read the everything else runs fine part?? Meaning other apps.
You're not alone, I also have the 1900XT and get graphic artifacts sometimes while playing WoW. Stock fan. Funny thing is, WoW on PC via Bootcamp doesn't have the same problems.
I've always attributed the problem to the game code. Then again I only get the artifacts after playing awhile, and the fan is running loud so it may be an issue with overheating. I'm still searching around, if I find anything I will post it here.
Here's a link to Blizzard's posting about Mac graphic issues
There's also a new patch released today, so maybe that addresses some of the graphic concerns.