2010/8/17 Maximilian Bräutigam <max-braeu@gmx.de>
You should backup all in / except
/tmp/*
/sys/*
/proc/*
/lost+found/*
/dev/*

Distfiles are saved outside the root  and I can afford to rebuild world. My 
main concern was losing (gentoo) config files, speaking of which, I 
remembered to back up /usr/src/linux/.config, and user folders. 

I have no solution how to bzip or gzip your backups or how to make a dvd
backup, but I use "app-backup/rsnapshot" which uses rsync but implements
an intelligent rotating system that is done daily, weekly, monthly,
yearly according to your config. Of course you should store the backup
on another physical hdd.

By the way, since a new hdd of one TB is pretty cheap, think about
running your gentoo in a software RAID. Guides:

http://www.gentoo.org/doc/en/gentoo-x86+raid+lvm2-quickinstall.xml
http://en.gentoo-wiki.com/wiki/Software_RAID_Install

Thanks for the advices. I am adding rsnapshot to my list.