Debian or
Slackintosh would be the best for something of that age,neither of those is really reccommended for the newest user, but with both have lots of documentation and i'm always looking for powerpc questions to answer
. It appears ( source wikipedia) that its a new world mac so any powerpc distro will boot on it, its just your choice now.
Other choices
gentoo
fedora
everything else seems just to be a fork of one of those, have fun