As a musician with a very little experience in amateur home recording, and, since you have a mac, I would look a firewire audio interface (mid-range, not pro, something in the 300-500 euro price range). But then you should look for a good microphone, and a cable, and you have to connect the sound card to the main supply... Too much hassle... Really. I think a good external recorder would be better for portability. Then, in your video editor, you will add your audio track and sync it to you video. Here you have to investigate by yourself. I know there are various external recorders dedicated to video but I have no experience with them.
Here I found something, hope it helps:
DSLR News Shooter | Audio DSLR Audio - Top 5 Best Recorders | DSLR FILM
Anyway, here's another interesting website,
NoFilmSchool, aimed to indipendent videomakers (maybe too much for me and you) where you can find a lot of info but there's also a free book called "the DSLR cinematography guide" that you can download freely after a free registration.
Ignazio