Questions to ask yourself:
Do you want it to have its own sounds when not connected to the computer?
When you use the word "piano", are you used to playing a 'real' piano?
How many keys will you need?
Will you need a 'touch' like a real piano, or is an unweighted, keyboard with no feel sufficient? (people who have spent a lot of time playing a real piano often have a hard time adjusting to something with fewer than 61 keys and with no resistance to the key touch)
For use with GarageBand, it is useful to have some knobs and sliders that can be used as controllers for the software.
Brands to research: M-Audio, Edirol, Novation, Emu, CME, Korg, Alesis, a few others I can't think of at the moment.