Doubting it, thought these were pci-express cards? Hm.. not positive.. but probably not likely either.
You guys don't look very hard This page (very interesting read) has a useful link at the bottom about the G5 9600s: http://www.macmod.com/content/view/74/2/
http://www.techseekers.net/modules.php?name=Reviews&file=index&rop=showcontent&id=71 that's your answer, five days and five responses later... that site shows how to install a G5's Radeon 9600 Pro into a G4. I believe your G4 has to have a 4x AGP slot. therefore this should work in your Quicksilver.