I hate to state the obvious, but if your onboard GPU is working fine (albeit an understandably crappy framerate), and you've done complete and fresh installs of drivers, etc., then you might look at hardware issues. Swap out the video card for another card you might have on-hand, see about borrowing a card to test, etc.

When all other options are exhausted... you gotta take what's left. Unfortunately, in this case, that might be hardware.