What type of interface are you looking for SCSI, USB 2.0, Firewire (Which is the fastest out their now)?
They range from 40 to 100 gigs per hard drive.
Than you also consider the speed of the hard drive, what are you going to use it for. 72000 RPM is the standard. If you are going to use it for video editing I would suggest a 10000 RPM or better.
For price, internal drives are about $50 to $100 less expensive and don't take up extra room and a plug. So if your computer can support an internal hard drive, I would get that. Otherwise you can go to mySimon
http://www.mysimon.com/category/inde...vesstorage&v=1
and they seem to have a wide sellection of hard drives.