@bloodline,
Garageband is pretty good, but you are not going to do any Professional work with it.
That might have been true in the past but now that Garageband is 24bit and can do 4 different real time effects per track at once I see no need to upgrade to Logic.
Click on the link in my Sig and listen to "Dancing Milk Cartons"
It was done with Garageband, AU plugins, a single MIDI Keyboard, and a microphone.
As far as the original poster goes though Gargageband cant do the type of MIDI setup he has going.
It all depends on what your writing and how you do it I guess?