If you're buying new you might want to consider doing what I've done with my new one.

I have 2 HD's congigured in "Raid 1" array, the advantage is that eveything on one drive is mirrored on the other, so if one goes down you can still use the other while replacing the defunct one.
The obvious disadvantage is in reality a whole HD doing nothing for most of the time, for me however the advantage of a fully working back up far outweighs this, as I know from first hand what a disaster a HD crash can be if all data is lost!!

Injustice will be avenged.
Cofiwch Dryweryn.