Here's how I do it:
-Logitech web cam
-Audacity (recording mic)
-Fraps
1. Start webcam recording
2. Start Audacity recording
3. Open game or alt tab back into it.
4. Say "Ready....START"
5. Press key to start Fraps recording right after you say "START". Then do your gameplay commentary.
6. In your editor, add in your fraps footage, your audacity file (exported to .wav), and your webcam footage.
7. Overlap all 3 things together and match up the audio from Audacity, and your webcam footage.
8. Once your webcam and audacity audio is matched up, cut out the beginning until right after when you say "START."
9. Align audacity audio/webcam tracks along with the beginning of the fraps footage and the audio will be in sync.
10. Delete your webcam audio track so you are using your audacity audio.