Look into Carbon Copy Cloner and SuperDuper. You can also use Time Machine if you are on Leopard. Read up a bit on them before using them. And if you do a backup, make sure you check it works first.
For Archive and Install, you don't HAVE to make a back up (but I'd recommend backing up important material anyway) -- you can tell the machine to keep all the settings and options intact. For Erase and Install, you have to have a back up, if you want to bring things back.