AT72 FLC RECEIVED A GPWS DURING A RAPID DSCNT BELOW MDA. VISIBILITY WAS LIMITED RWY REF TO APCH LIGHTS; ONLY.

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WE WERE ENRTE TO GSP IN POOR WX (700 FT; 3/4 MI) WITH LIGHT RAIN AND FOG. WIND WAS OUT OF THE NE; BUT THE ILS WAS OTS ON RWY 3; AND THE WX WAS BELOW OUR CIRCLING MINIMUMS (1000 FT; 3 MI). WE THEREFORE WERE OFFERED EITHER AN NDB TO MDA 770 FT OR AN ASR TO MDA 740 FT. WE TOOK THE ASR TO RWY 3. THE APCH WAS CONDUCTED FLAWLESSLY TO THE MDA; AT WHICH TIME THE ACFT WAS FULLY CONFIGURED TO LAND WITH GEAR DOWN; FLAPS 30 DEGS; REF PLUS 20 KTS. THE FO THEN CALLED 'APCH LIGHTS IN SIGHT;' BUT THEY WERE OFF TO THE L; 10 O'CLOCK ; AT ABOUT 1 1/2 MI. I FELT THAT WE WERE TOO HIGH; TOO CLOSE; AND NOT WELL ALIGNED. THE APCH LIGHTS WERE THE ONLY VISUAL REF. THE RWY AND TERRAIN WERE NOT IN SIGHT; AND I WAS LOOKING THROUGH RAIN AND FOG AT THE APCH LIGHTS. I TURNED TOWARD THE LIGHTS; REDUCED PWR ALMOST TO FLT IDLE; SLOWED TO REF PLUS 10 KTS AND DSNDED RAPIDLY. THEN THE GPWS WENT OFF (SINK RATE); THE FO CALLED 'ADD PWR;' I WENT TO ABOUT 50 PERCENT PWR; STOPPED THE SINK; AND LEVELED OFF ALARMINGLY CLOSE TO THE APCH LIGHTS (MAYBE 75 FT AGL)! WE LANDED NORMALLY BUT MAY HAVE SCARED A FEW PAX! IN REVIEWING THE EVENT; THE FOLLOWING CONCLUSIONS/LESSONS LEARNED WERE ESTABLISHED: UPON ARRIVING AT THE MDA; WE SHOULD HAVE NOT LEFT THE MDA UNTIL ALIGNED WITH THE RWY. THE FO WAS LOOKING AT A CHART DURING THE FINAL DSCNT. SINCE I WAS 'OUTSIDE' THE AIRPLANE; HE SHOULD HAVE REMAINED 'INSIDE' ON THE GAUGES. WHEN BEGINNING DSCNT FROM THE MDA; I SHOULD HAVE USED A NORMAL (GS) PWR SETTING PRECLUDING A RAPID DSCNT. ANY OR ALL OF THESE WOULD PROBABLY HAVE PREVENTED PREMATURE FLT INTO GND PROX. WHILE WE MAY ALSO HAVE 'MISSED' THE APCH; THAT WOULD HAVE BEEN A BETTER CHOICE.