Just from the thread title, I was all set to reply that I’ve learned that simplistic face modeling for animation is an all together bad idea, that for facial animation, the face mesh has to be pretty dense – but you mean really simple!
There are some good face rigging tutorials around (search at Vimeo and YouTube), but your model might be so simple that that stuff doesn’t apply.
IMO you should definitely model the face parts (left eye, right eye, nose, and mouth) separate from the body, and it depends on how you mean to animate them whether or not you want to use all bones, or bone-driven shapekeys. For example, is the mouth going to open realistically, or cartoonily (flat shapes, like :-D, or :-O)?
If they’re just going to be flat shapes, check out the articles on shapekeys on the wiki; and either way, if you haven’t already, go through the Introduction to Character Animation tutorial, search these forums for rigging tips (especially this thread), and deconstruct some of the openly available official and community-built rigs, to see how they work.
(Also BTW, this thread probably should have been posted in the Support > Animation board!)