BTW: adress book, export/import, right?
Export all your contacts as a single vCard file (get the com.apple.addressbook.plist file, too), then drop it onto the open Address Book after the reinstall.
Mail depends on what kind of accounts you have: POP or IMAP (or both). Generally, archive the mailbox(es) and then import them after all the updates for 10.5 are installed.
iTunes: definitely copy the whole iTunes folder, and the com.apple.itunes.plist file, then copy them back to the same locations after everything is updated.