G5 PCIX video card in a G4 tower?

The early G5s were PCI-x and had AGP video cards. No, they don't work in G4s because they are keyed the opposite way.

I can’t speak for other cards but I modified a GeForce FX 5200 AGP card from a G5 to work in a MDD. One thing I lost in the mod was ADC power. It's not a great video card but it was free.

One thing I lost in the mod was ADC power.

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I got such a card, except I think it's ATI, and there exists a way to "hack" the ADC power to connect to the G4's ADC slot which involves taking a chunk of ISA card of approximately the same size and soldering it to the cards misplaced connector and offsetting it with wires so that it fits into the slot properly.
I bookmarked it somewhere, and I'll post it up here at some point if I find it.

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EDIT: Also, pins 3 and 11 have to be taped off or disconnected somehow on some cards, otherwise they interfere with the G4's AGP slot, causing a no video condition.

Also take note of how your G4 AGP slot is keyed and how the donor card is keyed.

Were those AGP G5's AGP Pro (which can be confused with PCI-X because of the long card edge)?

Yeah, it was an AGP Pro slot. As I recall, the early cards are keyed for 1.5V.