For Murphy I always thought of Kristin Bell (Sarah Marshall in Forgetting Sarah Marshall). She has the look and stature (she's 5'1") for Murphy if you could get her toned up some to make the martial arts scenes plausible. The other suggestions I see for Murphy are good ones.

Harry is tougher. Jared Padalecki from Supernatural has the height and some of the look. His voice doesn't scream Harry though. For Harry you need someone tall and young. Best case with 20 books and TV series on say HBO/Showtime you'll be trying to go through 3 books a season. Which equates to 7 years and a wizard that shouldn't be seen to age that much. So someone in their mid, possibly late, 20's is what you would want.