I'm going from memory but I'm sure that you have at least 2 pcie slots on that board. There should be a toggle setting in the BIOS for graphic card priority: PEG/PCI or PCI/PEG. This controls which slot the BIOS looks at when it boots. I found this out once when with 2 vid cards in the machine I couldn't see the boot screens when the machine started, but when windows booted the graphics suddenly appeared.
You'll have to boot the board with a plain jane PCI card; I will assume that you have one since you say you've seen the BIOS. Look for that setting. It's a northbridge function, so take a look in that area of the BIOS. Not sure if it's the problem but it's something to try. I'm guessing it's the usual AMI BIOS template that ASUS uses these days.
It also may be good to do a long
CMOS clear before doing this; remove the power and put the cap on the clear header for a couple of hours at least. Pump the power button and the reset button a few times and let the box take a nap.