Make backups (CDR, DVDR, another drive, etc.) of your tunes. Nothing more heartbreaking than when 200GB of soundage goes pttth. Whatever hard drive you choose it will eventually fail; they all do, whether in 6 years or 6 months. That's why no hard drive manufacturer will guarantee your data. Unfortunately, I speak from bitter experience (45GB **poof**).
I have a smart playlist in iTunes that simply keeps track of anything added. When it reaches 4GB I burn the list to DVD and start a new playlist.