I looks like videochip overheating problem, check the video card fan if there is one...if not check from driver to see if you are running in overclocking mode, usually if you bump up your video memory too much you will see this kind of artifacts....downclocking my helps a little.....in worse case...change your graphic card....The inno3D at $149 should fits your requirement, I personally will chose the XFX 6600GT card because of the DDR3 ram, I know there are better cards out there but I will rather save that for future upgrade :lovegrin:
http://sg.hardwarezone.com/priceguide/priceguide.php?id=304