about Gentoo you can use the Gentoo Handbook for PPC :
http://www.gentoo.org/doc/fr/handbook/handbook-ppc.xml
i used it and followed it without any problems until it came to the kernel stuff, but all missing infos can be found here (see David Holm's posts about make.conf file, and about simplier way to build your kernel). I only had a few issues that got solved easily with the help of David and a bit of thinking from myself.
I have to mention that i am (was) a complete newbie about linux before that.
Debian installation procedures can be found on this board too i think.
good luck.
SSQ (writing from Gentoo on his peg now)
EDIT : sorry the link i gave you about Gento handbook is the french one, browse for your own language !