Skiz,when you get your Mac,GB willbe on it;in the newer Leopard OS-it's part of I -life.There are also a bunch of loops etc. from Apple that you can buy/load.That's why I wondered about needing both of them.Good luck-when do you get your Mac?
Chev,you can get a Midi to USB from Midisport that runs under $50;plug it in to your midi out/in on your MO into your computer,Whether you need a sound card or better card depends on your system.You have to load or get the drivers (should be a disc with the midisport)and then your midi uses your computer to record,etc.I suspect when your finished you can load your project to i-tunes and export it.
I haven't done a lot yet,but on my Mac I have a program called Bootcamp which allows it to run Windows XP.My Band in a Box program(xp) has some pre-loaded songs that are "takes" on say pop tunes.Since this has intros/breaks/chorus etc.,I pick the key/changes-what instrument playing parts;i.e.,drums,horns,guitar;trigger a Roland JV 1010 module for the sounds(thru midisport);and play along just like having a full band.You could recored/export,etc.
The midisport also works with;like Sz said a cheap midi controller-so I can use mine(or the mac keyboard itself ) in garage bad.