I recently replaced my PSU recently after my old one had fan problems. Its less powerful, (600w, down from 650w) but should still be well within the requirements for the system.
When I have my video card (GTX570) plugged into the PSU, the motherboard doesn't turn on at all. When I unplug it, the mobo starts normally. As I mentioned before, this new PSU should be well over the requirements (Which I have calculated at 450w.)
Are you certian you are not mixing up the CPU power plug(black, 4+2 pin) with the crossfire power plug (red, six pin)?
Other than that, I'd say one of the two rails on your PS is bad, making it a 300 watt power supply. I don't know of a good way to test for this, other than a load test, which it obviously fails. Stay away from OCZ, they are not a reliable brand.