I think you can use garage band to record straight out of the inbuilt mic. Theres this great bit of software for rerouting audio inputs into applications if you can't its - called 'detour' http://www.freemacware.com/detour/
You will have to install some rubbishy application enhancer thingy from there as well but its not problem really.
I use an MBox 2 to record mics through an old pre and a couple of guitars, a bass. If you want an outboard box, my only advice is go firewire, because USB has power problems with a lot of laptops including my Ibook G4, which makes it portability totally redundant if you can only use it on a desktop.