I had a similar problem, and after changing the mobo, and not getting a result, I RMA'd the graphics card. Solved the problem.
A few days after getting it working again, the pc decided to keep crashing to a corrupted video screen, and I had RMA the ram.
Don't know if there is any relation between the two problems, but when you fix it, probably a good idea to run memtest.
Hope you get it sorted soon, I know how you feel.