Just get a 1/4″ to 1/8″ adapter from Radio Shack. The Mac will take care of the pre-amp.
i would get a line 6 toneport gx. its only $50 and its got tons of amp models and effects. plus there will be no lag in recording.
For stereo and its twice as much then went into system preferences and its twice as much then went into system preferences and changed the effect if you dont like it in garageband you can never take them on in.