You might want to check out the Rocksmith "RealTone" Cable. It is supposed to be used for the two Rocksmith games, but I have heard people say it just shows up as a separate audio device when plugged into a computer (Windows or Mac). Amazon Link
Check out the first review. The guy says he got it to work with Garageband and it does the job well. Decent deal for $30.00.
Then if you like it, you can always buy the Rocksmith version that doesn't need the cable