Yes -- any MIDI compatible keyboard will work with GarageBand if you have a MIDI interface that has drivers for OS X. I highly recommend the M-Audio line of interfaces, since they're fairly economical and drivers are kept very up-to-date with new OS X releases.
I bought a MidiSport 1x1 for under $50 about 4 years ago for some audio applications in OS 9. I still use it today with GarageBand and everything works great. You can get the equivalent of this controller for under $40 if you shop around.
For more info on these sorts of issues, you may want to check out:
MacJams.com
This site has GarageBand-specific forums for these questions and also has helpful articles and various topics related to GarageBand.
Good luck!