You need the hard drive, of course. Ultra160 is the current flavor these days. You can get them in 7200, 10K and 15K spindle speeds. Faster the better, of course. You want to make sure you get a drive with the standard 68-pin connector. The 80-pin SCA connector flavors are designed to go into a hot-plug chassis usually found in servers or external drive arrays. If you get an 80-pin SCA connector, you will have to purchase an adapter to use it and those adapters can be flakey sometimes.
You need a controller. Make sure it's an Ultra160 controller if that is the drive you get. Adaptec and Tekram are popular and run around $200. You also need an Ultra2/Ultra160 cable. These can run you $30+ but if you buy the retail flavor of the controller, you usually get the cable with it. (OEM controllers usually do not include the cable)
Popular controllers are the Adaptec 19160 (only works with Windows operating systems) or the 29160N (works with everything) or the Tekram 39U3D. The current drive is choice is the Seagate X15-36LP which comes in 18GB and 36GB sizes.
