Congrats, Bet, I'm sure you'll love it!
Ross - I (think) I have one of the latest Macs (MacBookPro running Sabre-Toothed Cat or whatever the latest OSX is) and it does have a Firewire port. It's a 9-pin port though, rather than the 6-pin ports you find on older MacBooks and iBooks. So I need to buy a few new Firewire cables - nothing's perfect, though I gather these Firewires are very fast. The USB ports are no slouches either provided they're USB 2. The older Macs were USB 1 and these were painfully slow.
Ethenet is good if you want to operate two computers at same time, for example multiplayer shooter games etc., But you need to set up the ethernet connection and it takes a bit of time. the Firewire thing is just for accessing data on one machine from another, without any need for special set up. The second Mac should appear on the desktop of the first one as a Hard Drive icon (usually orange). But bear in mind an older iBook won't be able to use a newer Macbook as an external drive, though it should work the other way round.
Bob - I must check Garagband out more so. It's my impression though that to get the most of it you need some kind of hardware such as a plug in USB keyboard etc., Otherwise how do you effectively use instruments like Grand Piano without writing the song note by note?