I have a gtx 280 on an eVGA 790i that's worked just fine for a year or so.
A couple of days ago, whenever I played a game the screen would become a scramble of seemingly random colors after a few minutes of playing. Eventually, the computer just wouldn't reboot.
I let it sit overnight, and the next day it booted up, but the scramble problem was still there. This morning I removed and reseated the video card, checked all the power connections, etc and the problem actually went away for a few hours.
After those few hours, the screen went black and now the computer won't boot. I turn it on, all the fans run normally and what not. After about 5 seconds the GPU fan starts spinning at full speed and just keeps going. No beeps, no nothing.