I just bought a new Mac Mini to be my general office computer and server. For large music library BU, I bought from OWC two Icy Dock Firewire 800 enclosures, in which I plan to install WD 2 TB Caviar green drives. One drive will contain my primary music library, the other a clone BU (non-mirrored, but manual BU to avoid random corruptions in iTunes.) A good friend has this Icy Dock system with WD drives, and it is virtually silent. No fan, very compact size, and beautifully made--about $128 each. Icy Dock also offers hot-swappable trays so you can use the same enclosure for multiple drives ( I plan to use one for photography), saving space and AC plugs. I have found both OWC and Icy Dock ( a Calif. co.) to be very friendly and helpful to deal with.
Another plus for buying from OWC is all their firewire HD enclosures have very Mac-friendly chips for reliable mounting and unmounting, usually the Oxford chipset. The Icy Docks are quieter and more elegant than the OWC -made Mercury Alum. enclosures. I have read that the WD enclosures are not very reliable for Mac firewire mounting, but the drives are very good, and quieter than Seagate and other makes. OWC offers very competitive prices for the internal HDs too. I really like dealing with them.
For a bootable back-up up my general office files, and the iTunes database file on my Mac Mini HD, I am using a new portable firewire 320 GB OWC Mercury on-the-Go drive, USB 2 & FW 800. No AC needed-runs off FW or USB power. A very handy, versatile unit.
One other great find is SuperDuper! 2.5 as my Mac BU software. If you buy a license for $27 you get a lot of powerful additional features beyond the basic "erase and copy" function of the free version. The biggest premium feature I like is the incremental " Smart Backup" option, which dramatically reduces the BU time and wear on the drives. It efficiently builds a bootable a clone of the primary drive. The documentation and on-line support are excellent.
The link for Icy Dock model I bought at OWC:
http://eshop.macsales.com/item/IcyDock/MB559UEB1S/Rob