A 6950 is a top quality card and it requires a 500w psu be in the computer to power it and since Powercool is not a top brand name then I would say that the power supply is not providing enough power for that card. At this point you can get a power supply for less than a new card so I would try that and see if it will power it up without crashing and if not and you have to RMA the card then you will have a good decent psu for the card.
Unfortunately I don't have another graphics card I can try. I just took off the door of the case, and felt and listened to the PSU. A whine is coming out of it, lasting for about 2-3 seconds and pauses for one before beginning to whine again very quietly. Under a bit of strain, the whine gets louder. I'm a bit scared of how long this has been going on for unnoticed. I put this PSU into this PC until I got a better one, and after thinking it was doing the job well, I left it in. Wish now I hadn't. Was a very stupid move!! Will take the GPU out, and run the computer without it until I get a decent PSU put in.
Thanks so much for all your help. I'll let you know if the PSU was in fact the problem. Will order a new one in the morning hopefully