>PC Magazine has a how-to guide on building an external hard drive.
* For portability and ease of use, a 2.5-inch external drive works great. I installed a 40GB SATA hard drive into an enclosure with USB 2.0. It is extremely light and portable. You only need one USB cable to both power and data.
* If you want performance, buy a 3.5-inch HDD with an eSATA connection. This will allow for the highest transfer rates. For those of you with Firewire 800 connection, that is excellent alternative as well.
* If you want storage (for videos, music, etc) the external connector type is no so important. I suggest buying a an internal 5400rpm IDE drive. The storage to price ratio is excellent and are more than sufficient for storing and playing back of video and music.
When buying an enclosure, review the power requirements. Some will only draw power from a free PS/2 port. If you have a free PS/2 port then great! But if your PS/2 ports are occupied, consider buying an enclosure powered via USB, or has an external power adapter.