It all went according to plan apart from once the installation had finished and I restarted the screen went all weird and psychedelic. This is the solution to that problem.
Start up the PowerBook from your installation CD by holding down C after the chime.
boot: prompt type rescue.
You now need to go through the same screens about where you are, what language you’re speaking etc. that you went through for the installation. Don’t worry, it’s not going to reinstall.
Choose the partition where the root Debian system is installed.
Open up /etc/yaboot.conf in Nano by typing:
sudo nano /etc/yaboot.conf
Find the part that talks about the image that you’re booting from.
If there’s a line that starts with the word append then you need to add this command to the rest of the commands inside quotes.
If not then create the line. It should end up looking something like this:
image=/boot/vmlinux label=Linux read-only initrd=/boot/initrd.img initrd-size=8192 append="radeon.modeset=0"
Save (ctrl+o) and exit (ctrl+x). Then type the following command to copy the config file across to the boot partition:
sudo ybin -v
Finally, type exit and they system will reboot and everything will probably by OK.