If you are willing to flash a card and also willing to dump that ancient ADC display(replacing it with a Dell Ultrasharp or something else good) then you should be looking at this:

http://themacelite.wikidot.com/compatibility

Otherwise, OEM G5 cards are a No Go.

http://www.xlr8yourmac.com/Graphics/FX5200_Mods_for_G4/FX5200_G4mods.html

You are essentially screwed if you want ADC.