you can backup all your config files inside /home/thenameofyouruser
it depends on what is important for you to backup, the files shouldn't be larger than 200 mbs
sometimes the thumbnails can make this folders grow bigger
remember that you can do a remastered version of the install by using mylivecd
i personally just make a backup of my bookmarks on opera or firefox and since i don't use thunderbird i don't backup that
the rest for me can be wiped and in 30 minutes after a new install i am good to go