I would read all of this article: http://www.data-backup-and-storage.c...ard-drive.html
It has a lot of good information about things to think about when looking for an external hdd.
Besides the things talked about in that article, there are other things to think about, one being how much money are you willing to spend on a hdd. Newegg has externals from $49 for a 60GB to $389 for a 1.5T hdd.
I would recommend that you do a little looking around and see what is out there, on websites, like newegg, tigerdirect, amazon. best buy. Then you will have a better understanding of what the options are for you, too.