Are you using the 'boot 3' resource in the system suitcase?
ELN is still working on a Mac OS ROM with the log disabled, you can make it go away, temporarily by hitting command+F2, but it will come back from time to time.
I think you PowerBook is a 6,8, not 5,6.
Does you Powerbook have a Radeon 9700?
I think I can make an 'NDRV' for it, so you can select the proper screen resolutions and color bit depth. I'll just need to know the " compatible" property of your card.
Boot into Open Firmware, then:
dev agp/@10/@0 .properties
Look for the compatible property.
Then to exit:
bye