I have a hd6970 and one day when i resumed from sleep mode the screen remained black. I rebooted but the screen remained black. I know the system was still booting into windows 7 as i can hear the windows chime music through the headphones.
I took the card out and cleaned it. Put it back in and still black screen. I tested a spare nvidia gtx 260 in the system and it works fine so i think to myself that the card is faulty. I take the 6970 to a mates house and it works fine.
The only power differences between the two is the ati has 1x 8 pin and 1x 16 pin molex the nvidia has 2x 4 pin.
I tried to spread the load across my new antec 750 w psu by using 2 different rails to power the one device but it makes no difference.
This is a card that has been working perfectly fine in my system for a long time. I dont think its a power issue as the psu is new and it fires up the nvidia card normally into windows where i can play for hours.