There is common issue with the r300 driver in Mesa and Radeon cards on G4 Macs. When you have KMS enabled and to see if useing opengl you get this.

rican-linux@ibookG4-gentoo ~ $ LIBGL_DEBUG=verbose glxinfo |grep render
libGL: screen 0 does not appear to be DRI3 capable
libGL: pci id for fd 4: 1002:4e56, driver r300
libGL: OpenDriver: trying /usr/lib/dri/tls/r300_dri.so
libGL: OpenDriver: trying /usr/lib/dri/r300_dri.so
libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: r300
libGL: OpenDriver: trying /usr/lib/dri/tls/swrast_dri.so
libGL: OpenDriver: trying /usr/lib/dri/swrast_dri.so
libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: swrast
Error: couldn't find RGB GLX visual or fbconfig

There are two ways to resolve this

1. set you default display to 16 in xorg.conf, but on PowerBook you really see the drop in depth bot so much in iBooks.

2. apply a patch found here

I would much rather do option 2 and test that, however I am having some difficulty learning how to apply patches and reinstall in gentoo. In debian or ubuntu i would apt-get source the package, apply the patch, then recompile and reinstall. Can someone point me to resource that can help. I really cannot find it in the gentoo wiki, but I admit it may be I am not search for the correct thing. Thanks!