Artifacts are usually a sign of either bad video memory or video memory thats overclocked to far. Is the video card at stock speeds? If not drop it down to stock. When I say stock I mean what the ATI stock speed is not what ASUS sets it at. Also check the heatsink to make sure its on the gpu correctly along with the memory if it's supposed to cool them also. Also check your voltages from your power supply.
I have a HIS x1950 pro agp factory overclocked by HIS. Had problems with artifacts and such, even on the desktop. Dropped the memory and gpu speeds to stock and have had no problems since. I didn't buy the card for the overclock but due to it exhausitng the air outside the case, so losing the overclock was no big deal.