I always try to make guitar cover videos for my Youtube channel, but my audio quality always sucks. I usually just use the webcam to record the videos and create the video in iMovie. I know a lot of people record audio and video separately and put them together, how would I do this? I have Snow Leopard software, an Epiphone Valve Jr. amp, and I usually use my RP70 for effects if that helps.
you're a stone fox
Obviously the mics on webcams aint designed for that sort of thing, they just don't have the audio fidelity required, You'd need a guitar interface device, (like a Line 6 GX or whatever) or you could just plug it into the input port, although I wouldn't reccomend it, its apparently not very good for the PC, plus you definitely get a inferior sound complete with a buzz.

What I do is use my interface device, set the webcam to pick sound up from that, play the backing track which wont be picked up, give my self a visible queue into my playing then line the backing track up with my playing. I'm sure there's a better way of putting the backing track on, but that's what I do.
The mic on Mac's is very bad at picking up distortion I've found, its good for acoustic guitars though.

Buy an interface + headphone/speakers
Plug your guitar in
Open up garageband so your ready to record
Open up imovie to film yourself with the webcam
Press record in both programs
Then import the aiff file into imovie and edit it over the video recording.
Voila!!!! Thats how I make some of my youtube videos, I prefer to use a better camera though
because the Mac isight is quite blurry/hazy.