Unless you've stored stuff in unusual places, if you back up /Users/you/ you should be fine (but I'd poke through the backup to make sure everything looks like it's there). All your "hidden" data (email, prefs, keychain, etc) are indeed in /Users/you/Library/
Note that if you have any applications that installed stuff "system-wide" (for example, pref panes available to the whole system, screen savers or fonts available to everybody instead of just you), those will be in the root /Library folder, so you might want to at least peek into the Fonts folder there (most of the stuff should be system fonts, but if there's anything you know you installed and can't easily re-install), though really if you're doing a complete re-install, you should probably reinstall stuff like that from fresh downloads anyway, and you'll have to reinstall apps like Garageband and Adobe CS2 from scratch, too.
One question: Do you really need to reformat? If all you want is a clean OS install, an "Archive and Install" would get you that without worrying about restoring files from a backup. Of course, you should STILL backup first, just in case, but its a time saver on the restore.
One more suggestion: If you do want to completely reformat, tou might try using Carbon Copy Cloner to clone your internal drive to an external, then reformat and reinstall the OS, then finally use the Migration Assistant to bring your old user (and whatever else) over to the fresh install--should work fine, and you can always re-clone from the backup or move stuff manually if something goes horribly wrong.
