one more vote for Carbon Copy Cloner.
if your external is the same size or larger than your tibook, then a complete clone is the best way to go. your external will then be bootable as well, in case something happens to your tibook.
after the first clone, you can do incremental backups of folders/files that you want to maintain. or, you can just keep doing clones once/week or whatever suits you best. i do complete clones because i like to back up all my library preferences, mail, and other stuff rather than just a single document folder.