A B737-300 flight crew was dispatched with a generator inoperative and the APU substituting. After takeoff the APU failed so the flight crew returned to land at their departure airport.

Narrative

We had returned to gate to MEL #2 generator as inoperative. APU was going to be used as power source for #2 generator. Weather at time was around a 400 FT ceiling with 5 miles or so visibility. Shortly after takeoff after entering IMC; we got multiple lights in the cockpit and heard relays switching in the CB panel behind us. We analyzed the problem and realized that the APU had failed; dropping the #2 generator off line. We continued our cleanup procedure and stopped climb at 8;000 FT AGL to decide what we needed to do. I decided that the most prudent action would be an immediate return to the airport. At this time we were IMC with 1 generator powering the aircraft. It was also determined that we were going to land the aircraft 11;000 LBS over maximum landing weight. We ran through appropriate checklist; discussed how the airplane was to be landed; and communicated the problem to the Flight Attendants. The only thing I think I might have done was declare an emergency with ATC. They gave us the handling we needed because we told them that we had a maintenance problem and would need an immediate return to the airport; but we were never given any priority handling. The touchdown and taxi back were uneventful. Great job by First Officer and Jumpseater in assisting me with all aspects of handling problem. May declare emergency in the future because I was down to 1 generator in the weather.