Alternately, you could go with an older, slower, possibly cheaper PCI video card as well. You could use a PCI video in combination with the AGP card. Not sure if the AGP card is missing, if you can run with a PCI video only.
ATI made a couple of PCI Radeon cards for Mac, 7000 and 9200, I think, with VGA & DVI outputs. Might be hard to find these days.
Aside from the first-gen Power Macs, which used NuBus, Macs generally had PCI slots.
The guy with a Power Mac G4 in the article definitely has PCI slots.
Can probably just put an old video card in the thing.
googles
Yeah.
https://discussions.apple.com/thread/7937922
I had a radeon 7000 PCI card back in the day, it was a steaming pile.