Sixmile,
the A320 family comes with wingtips, the 737-800 (and -900 I think) B]can[/B] have what is called winglets.
The diameter of the 777 engines is the same as the 737 fuselage.
The number of exit doors, which defines the max. number of seats which can be put into an aircraft, varies with aircraft length. There is a maximum distance between exits defined by the FAA and other authorities.
Cop ,
Two easily visible differences:
front view: 737 new generation engine cowlings are not circular, the lower part of the cowling is flattened to give greater ground clearance.
side view: the transition from fin to fuselage on the 737s looks like a triangle, on the A320s it's practically straight with a little curve.