Unfortunately I cannot give you the answer you are looking for. Both ATI and Radeon would like us to believe that it is a much more expensive project to make a video card for the Mac than it is for the PC. Actually, apart from the Firmware on the card, I believe they are identical! Without the Mac firmware and the necessary drivers, the card will almost certainly NOT work in your new machine.
There are web sites that will instruct you on how to "Flash" the firmware of a PC video card to Mac and how to get around the driver problem. Just do a Google search for them.
Enjoy the upgraded G4 and save up for a better graphics card
Hope this helps
So I am getting a dual G4 powerMac, 2.5GHz, system and finally getting rid of the bane of computing - Windows. I have some hardware that I will be using with my new Mac, such as my scanner and FireWire external drive (not to mention my iPod and digital camera). But my question is this, the unit comes with a 128MB nVidia card but my PC has a 256MB ATI Radeon 8xAGP card...can I possibly swap it with the one in the Mac, as this card has dual monitor out, DVI and so on. (There is no official driver support from ATI on their website though.) Any thoughts?