imovie does all that, easily and quickly. Good for dubbing over after too.
If you need help, tons of tutorials etc. about: http://www.apple.com/support/imovie/
Rough work flow:
1. Record all your video
2. Open imovie, file->import and import it into an event
3. Make a new project, choose any default template (if reqd, or none). then you’ll get your timeline up the top, and click-drag to select segments of clips at thebottom in the event view. You can scrub thru the clips just by mouse-overing them.
4. There’s a microphone button in the middle somewhere, that’s what you use to dub over after.