You will need:
-An external hard drive enclosure. I am using a Rocketfish IDE enclosure. Make sure your enclosure is built for the hard drive you will be installing, IDE or SATA. In this example, I'm using IDE. Installation would be the same for a SATA drive.
-The hard drive you are installing. I am using an IDE Western Digital 320GB Caviar Blue.
-A screw driver.