System will start with 5870, but not gtx560

Allright so first my specs:
Nexus 80%+ 800W psu
Core i7 950 not overclocked
Asus p6t motherboard
8gb of ram, vengeance 1600mhz (2x4gb) seated in a1 and b1
2x 640gb hard drives
Win7 64bit

Now to the issue, when I plug my radeon 5870 into the mobo, I can boot into windows and log in np, but when I install catalyst drivers etc, I crash in gui, except if I disable the card in device manager, resulting in me using the crappy on-board gpu. I first thought tyat the 5870 was busted so I plugged a brand new gtx560 in and it crashed at POST a few times, and then coming to the windows logo and crashing there. No error messages, and then I plug my 5870 (still no drivers) and windows boots up fine. I've re-seated everything multiple times, and checked wires and heat, nothing out of the ordinary. So is the mobo busted, or the psu, or is something conflicting with the drivers?

Replaced psu, works.