USB Audio Interfaces

I'm kind of a newcomer to recording, so maybe this is a stupid question, but I'm looking for a way to record my guitar onto my computer through USB. I've looked around and I've seen a couple of options:

Fast Track USB
http://www.m-audio.com/products/en_us/FastTrackUSB-main.html

Lightsnake USB Cable
http://sewelldirect.com/lightsnake-usb-instrument-cable.asp

GuitarPlug
http://www.mac-pro.com/s.nl/sc.2/category.334/it.A/id.1906/.f

There's a pretty big difference in price as well as form, I just wonder if I get a lot more benefit by having the actual box with controls on it instead of doing it through the computer via software.

Any help here would be appreciated.