Usually it's best to start off from scratch. So any important files and such, burn them to CD's or copy them to your desktop or whatnot.
Realy is the best and easiest way, IMO.
This guy
talks about installing debian on a Mac-Mini, which is going to be very similar to what you'd do on a ibook and such. (don't bother with making a custom kernel though.. that's only because the Mini is so new.)
here is one, doesn't mention dual booting, though.
http://www.ubuntulinux.org/support/documentation/howto/installation-powerpc
Here is a very detailed Debian install guide for PowerPC (have it in other languages, too.)
http://d-i.alioth.debian.org/manual/en.powerpc/index.html
Basicly you format, install OS X and as your installing OS X you make a big partition for Linux. After you install OS X you boot up with the Debian Testing 'netboot cd' and delete the partition that doesn't have OS X installed on it then you repartition that for the bootloader (very small partition), the 'root' partition (biggest), and swap (used for swap space for memory, make it a gig or so. 512meg if you don't have much disk space.). Then you install the base debian install, boot up on debian and configure and download the rest of the OS from online repositories.
I don't know if you'd want Debian, though. It's very polished on the PowerPC, at least for 'testing' it is. But Debian is designed for users already fairly familar with Linux operating systems and is sometimes hard for a new user to deal with.
Ubuntu is based on Debian and uses the same installer, but is more geared for new users and desktop setup.
Yellowdog Linux is from a company that is the ONLY official reseller of Apple products that install something other then OS X.
Gentoo has a PowerPC following, but is a pain to install and setup, IMO.
And there are a few others.. I've only used Debian and Ubuntu, so I don't know how the quality stands up on other Linux OSes.. but there are other popular ones that are just fine.