Well. Here's how I did it for $10. Even though I just bought the Yamaha THR 5 amplifier that has a USB-interface I can't use it since I still run OS X Tiger, bummer. So instead I connect the the Mac line in to the Phones socket on the amp. This means that the amp goes silent. I turn on "Listening" (not sure they call it that in the english version) in garage band, whatever comes into garage band can be heard through the speakers of your computer. Now, the speakers on the Mac book aren't particularly good, so I connect the "Phones out" on the Mac book to "Aux in" on my stereo set. I can now hear everything I play on the guitar as well as backing tracks, metronome, previously recorded tracks and everything else that garage band does, through the speakers of my stereo set. Cost, two cables around $10 and I get a fairly good recording.

Hampus