One important part of being able to edit easily is to have a smooth playback of the video preview. So having the fastest most powerful set up is best especially editing HD footage. one of the slowest parts of any system for moving data around is usually the hard disk. So fast internal hard drives are best. However if you do have to edit using external hard drives passing the data through Firewire is faster than USB 2.0. If you don't have a Firewire socket on your computer then it's pointless buying an external HD with one.
Most external hard drives have there own power supply are 7200rpm and having 1TB size is not much more expensive than one with half TB. Hope that clears some of the point for you.