The gist of it is that if it's new enough to support Core Image (which a Radeon 9800 definitely is), there are no OS 9 drivers for proper acceleration, as much as I hate to admit it.
I can think of a potential workaround, but it complicates things by adding a PCI graphics card to the system just for OS 9 use, while the AGP slot continues to pack something powerful with Core Image support for OS X.
This hinges on different vendors for the AGP and PCI cards, too; word is that enabling the ATI extensions with a R300-series card or later really bogs things down compared to just leaving them off.