DopeyFish said:
a midi keyboard is a controller.
True, but there are reasons why they make keyboards with built in midi functionality, and there's a reason why they make "midi controllers".
If you're REALLY into your music, and you're taking it seriously, you're probably going to want to invest in a controller instead of just using your keyboard as a controller. Why? Well to put it simple, controllers have a much smaller footprint, and can sit in you lap while you compose your work on whatever audio sequencing program you're using -- in your case Garage Band. You don't knwo how irritating it is to work on a song for hours on end, while standing or sitting in front of a monstrosity of a keyboard. It's easier to just place a controller in your lap, and go to town.
Midi controllers also have features specifically designed around inputting information into sequencing programs (ie pads, etc). I recommend a controller over a keyboard. If money is a problem, and this is just something you're playing around with or experimenting with, using a keyboard is fine. You can always invest in a proper controller at a later time.
BTW I recommend the controllers from M-Audio, fine pieces of work they are.