Hi,
I recommend using the search feature, as there are numerous threads discussing external hard drives until you drop unconscious by the thread side.
Just a hint: if you're on a Mac, why use the slow USB 2.0?
Get at least a Firewire 400 drive. They aren't much more expensive, but they are much faster.
Firewire800 is a must if you use the hard drive regularly for back-ups - and especially if you have your iTunes library on the external.