I recorded a five song EP for my band using GarageBand. I was using a mac Mini with 500MB of RAM. A pretty basic machine.
I did do some stuff with just plugging directly into the mac, but found that the levels were always very low. I ended up adding my old four track to the signal chain and used it as a mic-preamp. Aside from that...its a fine piece of software. I have since grown out of it, but I still use it from time to time, for simple, or quick recordings.