I've tried Yellow Dog, Xubuntu, Debian, and Mint with no real success - too much
time involved. The iBook G4's that I have are the old 800gh, 30gbHD so I really
needed a small footprint. I've have very good success with Ubuntu 10.4. Very
easy to load and configure networking. Obviously, it's not fast, but usable in a
pinch - plus, I just can't stand to send them off to the recycler.
On that same note, my old Mac mini's are very quick with Ubuntu on them.
With smaller hard drives, it's a good idea to depend on an external (HD or USB)
Hope this helps.