You read my mind renz about the PSU. I couldn't find another card to plug in but here's what I found interesting. When I plug in both the 8 and 6 pin, no boot (580 fan doesn't go on). If I leave the 8 pin plugged in only (pull out the 6 pin), the 580's fan churns and the computer boots. If I leave the 6 pin plugged in only (pull out the 8 pin), the computer boots (580 doesn't do anything). I think the PSU is not providing enough power. I thought 850W would've been enough but that may not be the case.
Anyone else think it's the PSU?