First off you don't need to disable the onboard video to use an add-in card. Worst-case the onboard is hard-wired to be primary in BIOS/DOS, but that's uncommon. Even in that case the system would boot up normally and the add-in card could be used in say Windows. Usually an additional PCI card will be treated as primary by BIOS.

So if your system doesn't boot with the card, either there's something wrong with the card(s) or with your system. You say you tried two cards. Are they known-good? If not, stick them in another system to be sure. If the cards are known to work, the problem is in the system. Have you been able to use any other PCI cards? Is the riser firmly in place?

Thanks for the reply and the info.I shall test the cards in another system to rule them out. I did try connecting an old wi-fi card as a test and it booted up fine to windows and I was able to install the drivers etc with no problems. I'll give it another shot and see if I can get it going.

Another idea is to locate the video BIOS ROM chip on the motherboard and if socketed, try to pull it out.

The graphics cards are definitely working. I gave them a test in a newer system. I tried taking out the ISA soundcard to see if that had some conflict or something along those lines but that made no difference. I'll take a look tomorrow to see if there is a socketed video bios rom chip. Thanks guys.

I have occasionally a few graphics cards that would complain and keep the system from booting if I didn't connect a monitor to them.
Maybe your BIOS always wants to use the onboard graphics card as the primary display adaptor and the card also keeps the system from booting if there's no monitor connected to it.
Have you tried to boot the system with the PCI graphics card installed but with a monitor connected to the onboard graphics card?