Are you sure that you have the most recent drivers for your card? I have the 8600 gt nividia card and I was also having some issues only when I had more than one client open. This started when I updated drivers from what I had been using which was the original driver that came with the card. I had to revert back to the original driver to stop the glitches. The other day though, I checked the nvidia site and there was another new driver available (sorry dont know the ver number, its a new driver since 2009 though) I installed this newest driver and have not had these glitches anymore.

One other thing that someone else suggested that may have solved my problems was to delete your config.wtf file . I did that after I installed the newest driver and have had no problems, so it could be the new driver or deleting the config.wtf that helped.