There is a site called http://www.thebadapples.info/ that is about Linux on
a Mac. The person who runs the site is Klaatu and he is a Mac user. His blog
is at: http://notblog.titradio.info/ He is sometimes called notKlaatu
because ther are many Klaatu's around.

I am not a Mac user, but have known about Klaatu and his podcast for years.
He knows his stuff.

I am not sure what processor your computer uses. If it is powerPC then your
choices for distros is limited. If it is Intel then you can run any distro.

http://fedoraproject.org/en/get-fedora-ppc
http://cruxppc.org/
Links: http://penguinppc.org/
http://www.debian.org/ports/powerpc/
http://www.archlinuxppc.org/
http://www.gentoo.org/proj/en/base/ppc/
http://sourceforge.net/projects/ppclinux/

see also:
distrowatch.com/search.php
It is rather dated. Ubuntu stopped supporting PPC a long time ago. SuSE
recently stopped support and Fedora will stop it after Fedora 13.