It's probably the video card. If you have an old one laying around that's compatible then try throwing that in there and see if it'll work. If not try borrowing one from a friend or just buying a really cheap used one, just to try to isolate the problem. It could either be hardware, the card may have overheated and fried itself or it could be drivers, but most likely it's hardware. Other than that, it might be the motherboard but i highly doubt it.
Matt
May 2006