The iPod is a sync'ing device, not a music repository. The music on the iPod should be present on your hard drive in ~/Music. If not, then you're using the iPod counter-intuitively!
I guess what I'm trying to say is that keeping all your music on your iPod and removing it from your hard drive is a bad idea. If you're going to get an external drive for your music, simply put ALL your music on the external hard drive and point iTunes to the new drive. Every song you have on the iPod should also be present on your hard drive for maximum ease-of-use... if you try to use iTunes and the iPod in a different way, you'll definitely run into problems somewhere down the road.
I realize that many people use their iPods this way, but it is unsafe (especially if you have over 50 gigs of music). If the iPod goes, bye-bye music! Instead of backing up the iPod, simply leave the music on your hard drive and use the iPod as it is intended: to sync the music. That way you'll always have a backup on the hard drive should the iPod take a dump. If you don't have enough space on your hard drive to keep all your music, then your idea for an external hard drive is perfect.