I have run linux on older low spec machines before. The biggest problem these days is the linux 3.0 kernel having compatibility issues with older hardware (especially 3D graphics acceleration). Linux 2.6 works pretty well so look for a distro that has it. I would stay away from xubuntu these days as it is just as resource hungry as standard ubuntu. Don't try to use any Live CDs from the heavier distros. The larger distros (ubuntu, fedora, debian, arch) have their own forums and some have subforums for Apple hardware. Try searching for information in those forums for someone who has installed it on your ibook before.
The best solution I have found is to start from a minimal install (command line only) and then gradually add only what you need.
In order: graphics drivers and Xorg, window manager/desktop environment, browser, etc.
If you have never used linux before, I would recommend cutting your teeth on a virtual machine hosted on a modern pc. This will allow you to learn your way around the system before trying to hack it onto something that isn't well supported. Practice using the command line to accomplish tasks such as installing software, downloading files with wget, using a text editor such as nano (easy) or vi (power user).
Don't give up. It's definitely possible and opens up a lot of software possibilities for modern software.