That's the same PSU you said you were using with your old 7800GS? If it is and it gave you no problems before then I would rule out the PSU, unless it got bad over time (possible). I still say it's a software problem, but I could be wrong. You should try running some benchmarks like 3DMark06 and AquaMark3 and see what scores you get (or if you suffer from instability, see if they actually run).
The motherboard I was using was fried along with it.
Im using a different motherboard and PSU right now because I had to buy a new mobo as well as a new case. the mobo doesnt work at all, Ill test out he PSU.
Motherboard is P4SD-LA, PSU is from Bestec, Model No. ATX1956D. Looks like 200W is max.