Athlon Phenom quad 2.7Ghz,ASUS MeN72D MB, matched 4GB DDR2s, 2 Nvidia 8500's SLI'd and about 750GB of hard storage, 500 watt PS, in a 6-fan case, 2 Acer 21" LCDs. Happens overnight almost every night. Also happens when I start a browser (both IE and FF) after other graphic activity such as playing a game, etc. Temperatures and voltages are all nominal on reboot. Started just after the fresh install upgrade to 7 from XP, but it was a completely fresh install, no migrating.
This terrible instability is very frustrating; I even recently lost Firefox completely during an upgrade of a FF component when the system suddenly rebooted. Had to uninstall and reinstall FF. There is no warning either, and I can't find any explanations for the crashing anywhere. So add my tears to the pool.