I don't think the acting Gladiator is over-acting. It's almost impossible to compare Gladiator and Traffic, two completely different movies in every way.
It may seen like they are overacting in Gladiator, but I think it comes with the nature of the piece, which is very dramatic, very theatrical and almost Shakesperean. When you watch Hamlet do you think they are over-acting?
Traffic sides on realism, the actors are more subtle. Doesn't mean that is good acting and the acting in Gladiator is heavy-handed, it's two different styles.