Sorted it, so I'll record everything here in case it helps others.
Installing on the second hard disk didn't work (couldn't get yaboot to boot it) so install on the first disk (sda). I wiped OSX completely.
Start with Lubuntu 13.04 live cd, boot with options:
Wait until the white open firmware screen appears and then 60 seconds after the cd drive stops whirring type:
live video=offb:off nouveau.modeset=0 single
(use your own monitor resolution and colour depth)
modprobe nvidiafb mode_option=1440x900-32
You should then get a single user black shell, if you don't you'll have to reboot and try a different wait time than 60 seconds.
In the shell type: This will get you to the Lubuntu desktop!
Open a terminal and sudo gparted to erase all the partitions on sda except the first one, (which is a partition map I think)
Run the installer using 'erase disk and install'
Once it's finished drop back to the single user shell (ctrl-option-cmd-f1)
First chroot into the hard disk file system
Then make the necessary mods to the boot files as follows, since you're in a root shell you don't need sudo:
# mkdir /mnt/hd
# mount /dev/sda3 /mnt/hd
# mount --bind /dev /mnt/hd/dev
# mount --bind /proc /mnt/hd/proc
# mount --bind /sys /mnt/hd/sys
# chroot /mnt/hd
add this to the file
(ctrl-o to save, ctrl-x to exit)
edit the lines starting with 'append' to read:
append="nosplash video=offb:off nouveau.modeset=0"
You should see output including the line about penguin pee (yes, really)
ybin -v -C /etc/yaboot.conf
You may have to hold in the power button to fully shut down.
It should now boot into Lubuntu!
If your sound doesn't work, edit /etc/modprobe.d/blacklist.local.conf to comment out (add a # at the beginning) all the lines that have a 'snd' in them. (on my system it was all the lines) and reboot. You can also install kubuntu with sudo apt-get install kubuntu-desktop.
Sweet. A few more years use out of a nice old machine!