An MD80 flight crew experienced an electrical system anomaly that resulted in loss of both PFDs and NDs; plus communication radios. They started the APU and regained normal operation; but decided to declare an emergency and return to departure airport.

Narrative

At about 1600 FT MSL after takeoff; the aircraft electrical system cycled rapidly and then off; with audible relay clicking. At the same time; both pilots lost both PFD and ND and it was impossible to transmit or receive on the communications radio. After a quick glance at the overhead panel we were unable to quickly determine the source of the problem; and the APU was started. I was flying primarily on the standby instruments at this time. Once the APU came up and on the line; we regained the radios and informed departure we had a major electrical problem and that we were unable to navigate the RNAV departure. We also informed departure of our intent to return. I also declared an emergency because of the overweight landing; as well as the loss of navigational ability also requested assistance from CFR. Flight Attendants were briefed; and the passengers notified of our return. We were given vectors and a shot a visual approach. Touchdown weight 141;800 LBS and the touchdown was smooth; less than 300 FT rate of descent. We rolled to the end of the runway and had CFR standby as we monitored our brake temperatures. Brake temperatures never rose above 200. I notified Dispatch that we were on the ground safely after declaring an emergency. We then taxied to the gate under our power; and changed equipment to continue to our destination.