Yes, the fist is a great weapon and you are right about using it on the right targets.
Any place where it is soft-- as oppose to where it can damage your knuckles and put you at risk of not being able to continue.
The nose, solar plexus, liver, and groin. (I would stay away from the nose due to the only reason that breaking it will be bloody which means increasing the difficulty of dealing with the situation if the cops show up.)
The stomach is not a bad target either-- http://www.youtube.com/watch?v=MciLrVWcP8w
(I think the punch struck the stomach, slightly lower than the solar plexus.)