Newbie question -- which files to back up?

[Oliver Iberien]

I'm running FreeBSD 6.0 on a home machine and backing up to a DVD Burner, 
probably using kdar, the dar archiver that comes with KDE. 

My question is : which system files to back up, along with my personal stuff? 
I'm used to using linux distributions that do your system backups for you. 
The capacity of the DVDs sets a practical limit on what I can reasonably back 
up, so I need to pick and choose, basically to make recovery easier should 
everything go south. Thanks!

Oliver