Did you uninstall the previous graphics driver before installing the new one?

I have been told that this is important. I have never bothered to do it before so you will need to find a tutorial...

Well then I don't think it is a cooling problem since the card ran fine for a while. That shouldn't effect boot up unless it the overheating shutdown thing on the motherboard was still triggered,...

Ok so you have booted it fine with the stock card since the problem started happening?

Does it boot when you put another card in? I don't see why a defective card would cause the problems that you are describing. It sounds like it isn't getting to the BIOS, at least with your fans...