I don't think you mentioned the CPU card.

Given the symptoms, my guess would be a faulty/poorly installed CPU card. Try removing and reseating it. If that fails, unplug the power supply from the motherboard, pull the battery, press the CUDA button and let the thing sit for a while.

Make sure all your PCI cards are pulled except one video card.

Pull all DIMMs except one which you know is good.

SCSI problems usually manifest as a freeze at the grey screen, but one gets a good bong at startup.

It sounds like you're not even getting a bong. Which is what leads me to CPU card seating problems.

That connector for the CPU card is a pain.