There are very many things it could be. I too just put together another computer and ran into some difficulties. Refer to the troubleshooting thread located in the stickies. IT helped me fix my issue.
For a quick bit of advice, I'd make sure you have everything plugged in correctly. Also the 6950 requires two power plugs doesn't it? Make sure you don't have just one plugged in..?
yes everything is plugged in correctly, i triple cheched, i just tried a different card, and still nothing