I always thought that the 747 was designed with the 'hump' at the front of the plane specifically to house more passengers upstairs. But I watched a documentary on the 747 last week in which the chief engineer on the project described that the hump was created purely for freight reasons, ie so that they could open the entire front nose without the cockpit getting in the way. The first 747's didnt have seats upstairs.

P.S any finally for the plane spotters out there, the quick and easy way of spotting a 747-300 versus the 747-400 is the introduction of the vertical fins on the end of the wings on the 400 series aircraft.