You can see all Powerbook models here:
http://www.everymac.com/systems/apple/powerbook_g4/index-powerbook-g4.htmlAll Radeon equipped models should work, but Titanium ones might lack some support (audio on some models).
Generally speaking, the faster the better (by support too).
Touchpad settings and gestures, and certain special keys do only work on models which have USB touchpad/keyboard (at least 5,6-5,9 models). ADB ones are missing some functionality.
5,8 and 5,9 are slightly improved compared to 5,6 and 5,7. They have better resolution, both memory slots are more likely to work, better optical drive etc.
MorphOS will use max 1.5GB of the installed memory.