|What about trying the new power supply with all the rest of the original hardware.|
As was pointed out above, you are passing up reasonable amounts of cash to get this system running.
Furthermore, if you don't replace with an HP board your copy of Windows probably won't work with the new motherboard.
It sounds like your original problem may have been a defective power supply that could have also ruined the motherboard.
How much is the motherboard you ordered and what is the exact brand and model? Or are you getting an exact replacement board?