Hi and welcome to discussions.
Unfortunately it partly depends on which version of the Radeon 9600 you have, although there can be problems with them all. I assume it is a Mac version rather than a generic PC version which won't work at all (unless flashed, i.e. had the firmware updated with the Mac version of the ROM).
There's a combined PC and Mac version with 256MB VRAM. On release this was advertised by ATI to work in all AGP x4 G4 models, including the Digital Audio. However, shortly afterwards a number of people experienced problems using it in the Digital Audio and Quicksilver models, whereby no video would be shown. ATI then released a support note stating these particular models weren't supported, leaving just the MDD and FW800 models. In short it's pot luck whether it will work or not. I've been lucky in that I have one in my Quicksilver and it works perfectly, but others haven't been so lucky. More info can be found here:
http://www.xlr8yourmac.com/feedback/9600mac_pcedition.html
There's also an AGP x8 version which was shipped with the Power Mac G5s which had 64MB VRAM (I think). These can be made to work in AGP x4 models, including the Digital Audio, but some pins have to be taped over, otherwise the computer won't boot. Some companies, OWC I think, sold this version already modified for use in the G4s. More info can be found here:
http://themacelite.wikidot.com/pins-3-and-11
So.. I guess Apple support are right, it can take the 9600 and it can't!