THE ACFT WAS ON THE RETURN PORTION OF A ROUND TRIP FLT, HAULING CANCELED BANK CHECKS. WHEN THEY BOARDED BEFORE TAKEOFF, THE PLT'S WIFE, WHO WAS NOT A PLT, OCCUPIED THE R FRONT SEAT & HIS 2 DAUGHTER'S, AGES 4 & 6, WERE IN THE PASSENGER SEATS. PREVIOUS LEGS OF THE FLT WERE ACCOMPLISHED WITHOUT INCIDENT. WHILE EN ROUTE ON THE 4TH LEG, A RADAR HANDOFF WAS MADE FROM JACKSONVILLE TO ATLANTA CENTER AT 0225 EST & THE PLT REPORTED AT FL180. THIS WAS HIS LAST KNOWN TRANSMISSION. AT 0230, THE ATC CONTROLLER NOTED THE ACFT WAS PASSING MACON VOR. SHORTLY THEREAFTER, RADAR CONTACT WAS LOST & EFFORTS TO MAKE RADIO CONTACT WERE UNSUCCESSFUL. SEVERAL PERSONS IN THE VICINITY HEARD THE ACFT ENGS ALTERNATELY AT HI SPEED, THEN DEMISHING, BEFORE THE ACCIDENT. THE WRECKAGE WAS FOUND SCATTERED OVER A 2 MI AREA. AN EXAM REVEALED THE R WING HAD FAILED UPWARD & THE L WING & L HORIZONTAL STABILIZER HAD FAILED DOWNWARD FROM OVERLOAD. THERE WAS EVIDENCE OF COURSE REVERSAL & DESCENT BEFORE BREAKUP; LIGHT TURBULENCE & ICING AT CRUISE ALT. REASON FOR DSCNT UNKN.