Numerous Hard Freezes Computer Won't Boot

about two months ago, my gtx275 video card displayed green artifacts during a game and hard froze my system. i restarted the pc, continued my game, then about 5 minutes later it happened again, only this time it was impossible to boot after restart. when turning on the pc, all fans and leds were working as usual, including the one on the video card, but there was no display on my monitor and the computer wouldn't boot at all (no short beep, no nothing, just a constant black screen). i had another video card around, an 8400 gs, so i tried replacing the gtx275 with that. the system worked fine with the second card, but not with the first one. naturally, i thought the gtx was broken, so took it to the service (warranty), to be repaired. while gtx was being in service (took about 1-2 weeks), i've been using the 8400gs on my system. during this period, my computer would hard freeze as often as once every 15 minutes. happened most during any game, happened less while doing a random thing, such as watching a Internet video, and happened least when my pc was idle