Welcome to the joys of pcs
Just FYI, Im using a crt, and I havent had any blackouts since my upgrade
Im guessing because, 1 - the A64 uses less power than my previous Varton, and 2 - this 7800 may use less power than my previous 5900XT (not 100% sure on this). Also my blackouts only happend when my pc was idle also, hense taking a guess that our problems maybe the same. Im taking a guess that my problem was due to the PSU, never looked into it though as I dont have any quality PSUs just laying around
but I am defently upgrading my PSU when I go SLi as I dont want to take any chances. Are both PSUs you have tried no name? Do you know anyone that has a good quality PSU that you can try? I wouldnt want you to go and buy a new PSU only to find out that it wasnt the problem. The reason that Im thinking that it isnt your card, is that I havent read many people having this problem, and with the amount of nvidia cards sold, you would think that if it was that card, you would see more people posting about it.
Trouble shooting pcs can be a pain at times, one time I thought I killed my Barton 2500+, left it out of my pc for a couple of months, put it back in and it worked! Sometimes it can be as simple as a bios setting, like wake-up on LAN being enabled etc. Have you tried the card itself in a different pc yet? I would do that before sending it back.
Also, just noticed that you are overclocking, try running everything at stock just to see if it helps, as usually when OC'ng the cpu we raise the voltage, but not so when OC'ng the videocard, unless you flash the cards bios.