I know this may all be evident, but a few other things you should try:
-Make sure the hard drive(s) is/are firmly and properly connected.
-Do the same thing with your PCI cards, etc. as you did with the video card to make sure everything's seated properly.
-Absolutely make sure your memory and CPU are seated correctly, because this mistake can be easily overlooked.
-Try skinning down your system to the bare minimum components, and slowly work your way up until everything's in.
Hopefully that will help. If not, then it could be a faulty component (i.e. motherboard, CPU, etc.).
--Francis