Definitely go for the aluminum enclosure. Make sure the one you buy fits a 3.5" drive. They make them for 2.5" drives too. You also have the choice of getting a bigger enclosure that will also fit a CD-rom drive. It will come with the spacers to make a regular 3.5" drive fit. Personally, I don't like the big 5.25" enclosures (for a CDR) but you may.
I have
this one and really like it a lot. It's got an 80mm fan on the bottom; much quieter than the typical 20mm little buzzsaw you see on a lot of these enclosures. You don't need one w/a fan if all you do is back up the data and shut the enclosure down. But if you leave yours on for extended periods of time, it's advisable to have a ventilated enclosure as opposed to sealing the HD up in a box.
I've had my DS3 for just over a year and it's running great. Highly recommended.
ps
Be sure you buy one with the INTERNAL INTERFACE you need. I.E. IDE or SATA. And make sure the EXTERNAL INTERFACE is USB2.0 or FW if you have that capability and prefer FW.