I would suggest using Logic Pro. This will make it easy to start with your rhythm track, add the guitar track, mix it and create a high quality finished track. I would then bounce that from Logic Pro as AIFF to use in iMovie.
If you have a camcorder, use 48khz in your Logic project so this will match the sound from your camcorder, which is 48khz. (44.1 will work, but may be more challenging to sync.
I also suggest using a camcorder on a tripod. This will give you the best picture, and will be easier to sync. The built-in iSight or FaceTime cameras on a Mac are optimized for webcam and FaceTime usage. Their aim is to minimize bandwidth. So if you are not moving very much, they may shoot 12 to 15 frames per second. Then when you are active, they might shoot 25 to 30 frames per second. This variable frame rate means that it will be very difficult later to sync up your high quality sound.
At the beginning of your performance, use a hand clap or a slate so you will have a spike in the sound track of both the camera and the Logic Pro track. You can use this in iMovie to line up the tracks so they are in sync.
I used my external FX box with drum machine ... my seperate audio interface has 2 mic/instrument inputs and 6 Line inputs. I can plug my bass into 1 mic/instrument and the FX Box (drum machine) into Mic/instruemnt 2. That's fine, I can play along to a drum pattern, and both are recorded, audio and video.
So now I have a song I want to video myself adding the bass track to. I can creat an MP3 or M4a or whatever in GarageBand or Logic, but I still can't understand how to put it in the iMovie as an existing track I want to play over. If I drag it into the area, it just doesn't play .... I'm sure I use to do this with iMovie HD and it's Audio Timline ...iMovie '11 is so different ..or is it? Does it have an audio timeline? Where, how do I see it?
So I've created this track without bass. If I add it to the Event or Project Library, it might play back, but it only records the bass. I'm so frustrated .... I'm sure I knew this. Feel so stupid ..is iMovie '11 meant to be easier than iMovie HD? I don't think it is ....
p.s - I think that years ago I would create the song in GarageBand or Logic without bass. Then open iMove ... go back to the audio app and begin playback with count-in ... switch back to iMovie (or Photobooth?) and hit record. THen all I needed to do was trim off the beginning where I await the track to begin...