I would try Yellow Dog Linux. It was made originally made for the Power PC iMacs.
[url] http://www.yellowdoglinux.com[/url]
Ubuntu is also made for Power PC but it is an older edition.

There are many distros that will support PPC (what you'll be looking for for this particular machine.) but I have to get a plug in for the PPC version of slackware, slackintosh:

http://slackintosh.workaround.ch/index.html

I want to be able to post a topic but this forum won't let me! I am having an issue where I have gotten arch linux ppc to install but after install I get the error saying it can't mount my root partition which is /dev/sda4. It doesn't create the device node. The live cd still picks it up though? I think I must be missing some modules/hooks in my mkinitcpio.conf but I don't know what they would be. The computer I am using is a powerbook g4