I think that one element of becoming a "classic" is how the film holds up over time. The Godfather was obviously a great film when it was released, but the fact that it has stood the test of time and new generations embrace it makes it a classic. I don't see Road to Perdition following this path.

Another mafia classic in my opinion is GoodFellas. Not only was it a great film, but it is still talked about in circles such as this message board.