|"It's about 5 years old, and AMD Athlon 2500XP with a DFI LanParty NFII Ultra B mobo.... "|
I looked at the manual for that mboard - Copyright 2007 - it might have been made in late 2006, but it wouldn't be any older than that.
Software probably can't cause what you're seeing, but a poor connection in the AGP slot could.
You could try removing the AC power to the case, removing the card, cleaning it's contacts, installing the card, make sure it's all the way down in it's slot, install the screw that holds it down, restoring AC power, trying the computer.
"I did find one in my bits box but it was VGA only and I couldn't even identify the bus - it wasn't AGP or PCI and definitely not PCIe..."
AGP Pro cards were made for a short time. The contacts have two levels of staggered contacts like regular AGP cards, except there are more contacts. You can install regular AGP cards in a AGP Pro slot, but you can't install an AGP Pro card in anything but an AGP Pro slot.
E.g. AGP vs AGP Pro
"Maybe I'll track down an old Radeon 9800 or something "
Why would you want to install a card with a chipset that's a lot older than the mboard? Even if you don't want to spend much money you can get a new video card with a lot newer chipset than that.
The AGP slot only supports 4X/8X cards.