Actually... completely speaking out of my posterior, you *might* be able to get Core Image using a flashed GeForce 5200FX. (or possibly better.) The AGP-bus check affected "Quartz Extreme" cards, but the rumor at least is that if a card looks like it's Core Image compatible that check isn't enforced.

Of course, you'll ultimately be limited by the bus speed of the card. I've never even heard of a PCI-X video card, so the very best you're going to get is 66mhz 32 bit PCI transfer speeds, same as a B&W G3.