Well I finally figured out the problem... yes, it was the power supply. I've replaced it with a model that has dual +12V rails, and now I can run the BFG NVidia 6800 GS card I that was causing the exact same system power failure I was experiencing with the previous eVGA card.
But this story does not have a happy ending. In fact, the cure has been worse than the disease.
This card has so far destroyed my ability to play 3D games. Whenever I load one up (Sims 2, Half-Life 2, even the NVidia demos from the install disk), I get brief 5-second pauses of normal video operation betwen LONG 10-20 second pauses of everything freezing. This has happened with the drivers that shipped with the card and the latest drivers from NVidia.
I spent nearly two hours in the phone queue for BFG tech support last night before I hung up.
Will be trying to reach them again this morning. If a solution can't be found, I"m RMA'ing this one back too. And then I have no idea what I'll do.
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BFG tech support ran me through all their solutions including a complete driver scrub and re-install of the card. Didn't help. They want me to send the card back.