thats quite an old mac. 3-4 years.

the connector technology has changed several times in that timeframe.

from pci to agp and so forth.

the graphics cards that ati currently sells are listed here:

http://shop.ati.com/searchresults.asp?dept_id=31

you might be able to use the radeon 9000 for mac g4 only - this is an agp card.
or you might be able to use a straight pci card- the 9200- i have never tested with this card.

you might email or call ati and ask them which card is compatible with your old g4. its such a long time ago - i cant remember.