Comment 41 for bug 1541082

"r128" is the appropriate driver for your card but obiously does not work properly on your hardware. It would be a good idea to let the upstream devs know that. xorg/driver/xf86-video-r128 is actively maintained and still receives patches (last one 8 days ago). You could file a bug against upstream: https://bugs.freedesktop.org/buglist.cgi?quicksearch=r128 Just be sure to have a complete Xorg.log ready to post this bug upstream and state that it happens on an Apple PowerMac 3,1.

The "Invalid ROM contents"-line does not matter very much, it seems pretty standard on PPC-Apple. My Powerbooks are working fine also though this error shows up.

In the meantime you could try to get one of the other drivers "modesetting", "fbdev", "fbdevhw" to work with 24 bit depth. You can use my xorg.conf as starting point and edit it manually. Just put it in /etc/X11.