Head Switching is the general thought of what you're describing - even though you only mention the face specifically.

There are any number of books and places you can learn to do this well - I'd start with:

I've Got a Human in My Throat
http://www.amazon.com/Ive-Got-Human-Throat-Delusions/dp/1598630709/ref=sr_1_1?ie=UTF8&s=books&qid=1242385418&sr=1-1

There should be a link to "When Pancakes Go Bad" and Surreal Digital Photography" below as the "up-sell" Amazon always goes for.

For general Photoshop knowledge and advanced training:

www.photoshopuser.com

Yes, I pimp this location big time - almost as though I own stock or something - but it's worth it. If you can afford it, get yourself a membership ($99 a year), which will get you a subscription to the magazine as well, and you will have so many tutorials available to you it'll make your head spin.

Any area in Photoshop you wanna learn, you will.

Hope this helps-
-Lew