I would certainly back up my home-directory, but perhaps not the ~/Applications folder (if you have one). ~/Documents, ~/Library (for settings, e-mails and so on), ~/Pictures etc. Maybe some of the scripts in /Library/Scripts (if you have created some), or the Apache folder /Library/Webserver (if you have Apache up and running), or some of the apps in /Applications. If you have OS 9 on the same partition I would backup what you have installed. The rest will pretty much be re-installed. But again, it depends on what you want to save.