Today PC with same problem came tu my shop. GTX 480 was involved and did not work. Neither my PCI E test card worked. So did basics (disconnect 24pin and reconnect, remove one stick of ram), turned it on and voila... it worked. Good thing is that i have always spare speaker to connect onboard. In this particular case no beeps were involved when video card inserted. So it means that comunication between controllers is not established. It means that something on board is stuck. Since you narrowed it to GPU it is probable that your problem is same.