A comment about GarageBand
I was playing around with GarageBand just now and noticed something annoying. When I activate/deactivate the mute on a given real instrument track, there is a HUGE lag between clicking the mouse and the audio being output. In other words I click it, and then don't hear it for over a second much of the time. There are only three tracks.
Ardour does way better than this on my machine; OS X, 64 bit dual core, 4Gb RAM. I experience no such lag in Ardour.