I'm not an expert by any means but I've done a number of music videos using the same software/hardware you mentioned. You can get what you want. There's an easier way and a harder way, but either way, you will have tro sync the audio to the video inside iMovie.
The easier way -- record the music the way you want to in Garageband. Mix/export to iTunes. Close Garageband. Open iMovie. Start a new project called "Whatever lYou want." Push the camera button and shoot you playing the way you want (WHILE YOU"RE LISTENING TO THE FINISHED MUSIC TRACK on iTunes). Play well. After you're done shooting the video, open the "I" button in iMovie which is an import window. You can get the finished track in there and drag it into iMovie. Now, sync your track up to your video - LIP SYNCING it's called.
The harder way -- Basically it's the same as the easier way, the difference is you shoot the actual recording of the music instead of playing to the finished music. You will be shooting a very long time to do it without making musical mistakes. Instead of shooting from inside iMovie, use photobooth -- it's the same camera and all. But, in the end, like I said at the beginning -- you have to sync the finished track up to the video inside iMovie. But, the harder way will give you perfect syns where the easier way will be close, but not perfect.
