If your machine is AGP graphics, you buy a Mac compatible AGP graphics card.
If your machine is PCIe graphics, then you buy a Mac compatible PCIe graphics card.
Pretty simple stuff, I'd say.
YOU said your machine was an AGP machine AFTER I said that the fastest AGP card was a Radeon X800 or X850XT.
If the card didn't fit, then it isn't AGP. THAT'S not my mistake.
BTW, the fastest PCIe graphics card possible is a flashed Geforce 7800 GTX. The PCIe X800 Pro or whatever was pretty slow and couldn't be used in a Mac, flashed or not......
Oh, you don't have a PCIe graphics G5..........right?
Now you say that OS doesn't have anything to do with "it".
Well, Mr Knowitall, OS has everything to do with every focking piece of hardware in every focking machine.
If there are no drivers for the hardware in the OS, the hardware is worthless.
If the OS uses OF or EFI or whatever, a piece of hardware must also use that protocol in it's ROM.
IF you don't check OS compatibility with a piece of hardware, YOU are the idiot.