I don't think you even really have to see any. Just a quick synopsis would probably get anyone to the point where they'd need to be. The two I'd say would benefit the most from seeing are Captain America and The Avengers in that order, but I think anyone can enjoy this movie as a stand alone with just a little bit of info on who everyone is and who SHIELD and Hydra are.
Her performance was the one thing that bothered me in this movie. I don't remember feeling this way in Avengers, but maybe she didn't have enough screen time for me to form this impression. She was way too everywoman to play Black Widow. If she were playing the manager of an Applebee's in Delaware somewhere she would have been perfect. As a former Russian KGB agent and big time player in the International spy game she just fell flat.