Just one additional point to Kozikowski’s post above,
If the tracks are the same BPM you can adjust their position using Audacity’s “time shift tool” so the beats of each track coincide, (are synchronised), during the fade.
Youtube example of Audacity “time shift tool” use … http://www.youtube.com/watch?v=rvRUgVh8gfU#t=2m22s [thank God for deep linking]