You can use the Archive&Install option when Installing Tiger. It will basically erase the old OS and keep your accounts/settings. No need to backup on a different drive.
If you really want to do a clean install, copy the home directories on your external harddrive (go in /Users and drag&drop home directories on the external harddisk). Then use the Erase&Install option when installing tiger and create the users. Once this is done, you can copy the files back from the external harddrive.