At this point, it could be anything.
Do you have software that can monitor the GPU's temperature? Some of what you are experiencing could be heat related, especially if you are able to shut down your computer (after experiencing the artifacts), reboot after it has cooled a bit, and everything is OK, and the desktop is fine and your catalyst controls display properly.
Just a guess on my part, but I suspect heat. As an aside, is your power supply sufficient to handle the upgraded cards requirements?