Capacious hard drives are dirt-cheap these days, like $90 for a 1TB Hitachi 7K1000.B. These are great for rotating backups (multiple offline and/or off-site copies of your data).
The smart money is on the Mac Pro (see my special report), which can take 4 internal drives. However, bare hard drives in conjunction with something like the Voyager Q make a great backup alternative for any Mac. I do recommend eSATA for speed if you have a Mac that can use it, like the MacBook Pro.