A few months ago I bought a secondhand MSI facotry overclocked Nvidia 9600GT card to compliment my old one, of the same brand and model. I also purchased an SLI capable PSU (by APEX). Things were working fine for a few weeks.
Then while I was working in a 3d modeling program, I got would sometimes get pixelated "snow" artifacting, and it looked like static. All of my games experienced various artifacting issues, to varying degrees of seriousness. Furthermore, when using Google Sketchup, when I moved my mouse cursor around the card (s) would make a fast clicking sound.
I took out one card, then the other - same issues (though not as severe). I tested both PCI-e slots, and reinstalled Windows 7 64bit. I still get occasional artifacting running only a single card, but the bigger issue now is that gaming applications will very often freeze and become non-responsive. Interestingly, if I open task manager, this usually unfreezes the game. I still can hear a slight clicking sound coming from the card whenever running a program that uses graphics.
Sometimes, the whole screen will go black for a bit, and then I get a popup stating that the display driver has stopped responding and restarted. This happens maybe a few times a day.
I have run hardware monitor several times: temperatures come back normal.
So finally, suspecting that the PSU was responsible for it all, I switched back to my old one. However, it had no effect. I really don't know what is going on at this point.