I do get artifacts in GTA4 but for 95% of the time it is okay. i will try to get a screenshot of the artifacts in gta4. other games like CSS ans Garry's mod are un-affected. 3DMark06 is also un-affected, i've run it for hours over and over with no issues.
Speaking of 3dmark06, each time i run it my score seems to go down ~50 points.
I had a similar problem with my HD4870 and it turned out to be the overclock on my ram. Had to increase voltage to 2.1 to run @1066Mhz. It would sometimes artifact in gameplay and rarely give the BSOD.