Annoying issue with X1800XT, software related I think

Well I'm playing oblivion on high settings with 4xAA and it's looking great, good framerates and little stutter. Then I go to talk to some fool, on the zoom-up to the face my pc simply shuts off and restarts itself. It's not the first time, the other time it froze for a bit and then gave me a blue-screen.

Card isn't artifacting horribly or anything so I'm not too sure. I think I've seen the odd artifact, but it looks like a small texture glitch where 2 corners joined.

Got the latest Catalyst drivers, which I know are complete crap, but I have them. It looks like something is telling the sytem to shut down after my card reaches a certain temperature or is under a certain load?

Mobo: Asus P5W DH Deluxe
Processor: E6300
Video: X1800XT 256MB
Memory: 2gb corsair PC-6400, running underclocked at PC-5300, need to know how to change back to 6400
PSU: Seasonic S12 600w PSU

SEEMS to only happen when I'm using AA, I dont even get an error message. The sytem just restarts