AFAIK it depends on voltages supported by the card. Some 9800 cards work, some doesn't. Check out morphzone to see if somebody had one and can tell you brand&model.
R400... I don't think these are supported in MorphOS yet even in 2D. The most modern core publicly shown is Radeon x600 PCIexpress on iMac G5, and that uses a R300 core.
Yeah, I talked to Frank about the R400 several months ago. At first, we weren't even sure that R400 based cards would work in a G4.
I re-flashed a FireGL X3 with a Mac X800XT BIOS and taped over pins 11 & 3 on the back side of the connector and when installed in my Quicksilver it works fine under OSX.
So we can assume that re-flashed PC video cards (R400) could be supported by MorphOS (even without G5 support).