I suggest rippin the whole thing to bits so all you have plugged in is the mobo, RAM, CPU, GPU and PSU....
Then mouse, keyboard, monitor.
Then Clear the CMOS just to make sure there aint no crappy settings stored into the BIOS. (Use jumper, battery, what eva...). Then make sure the G-Card is plugged in, might be worth giving the AGP/PCI/PCI-E slot a good blow and even try a small clean brush to make sure the slot is clear.
Do same with DIMMs, then try that. Also do you have any spare DIMMs and GPUs kickin about just to test on the PC?