A friend of mine used to build computers for his boss. I stopped by to see him one day, and he had problem getting one booted up- he'd swapped out everything, as it unfurled, except the power supply. I asked him if he had another one, and when he swapped it out the computer booted and ran perfect. Other than checking all your connections, I guess the video board could be suspect. Here's a blog about this exact problem:
http://forums.pcper.com/showthread.php?t=446077
In a similar problem, the board turned out to be faulty:
http://www.tomshardware.com/forum/256259-30-4pin-8pin-motherboard-fails-post-foxconn-946gz7ma-8ks2h
Good luck, and hope this helps you.