I have a Western Digital 300GB USB2 drive that is awesome. It ran about $200 a year ago. You just need to make sure you use a USB2 card, and do not put it on a hub, plug it directly into the card.
I highly suggest buying pre-external drives, and not doing it yourself. So many things can go wrong, and often do.
Seagate and Western Digital are the only two to look at, forget everybody else.
Want a good deal? Look at Fry's Electronics or the online
www.frys.com site.