If you are serious about doing recordings you might want to look into getting an MBox, by digidesign. It seems you will be using real instruments as opposed to software synths and loops, so that means microphones. You will need some way to get those signals into the mac. The MBox comes with ProTools LE software, the defacto audio production tool. Sure some studios use Logic, and I use Digital Performer at home, but in the long run ProTools is king. At around $450 it is not a bad deal either, but it only handles 2ch of i/o over USB. The 002R runs about $1100 and will give you 8ch of analog i/o and 8ch digital over firewire.
I haven't used garageband v2 at all, but I find that while all of the apple iApps (including GB) do function well, they are at times too simplistic for me to even understand. If you get into wanting to be seriously creative then you will probably outgrow Garageband pretty quick. It is worth trying though if you have it already and are new to recording, but don't expect it to do very much.