MD80 CREW EXECUTES GA AT ATL AFTER FORGETTING LNDG FLAPS AND CHKLIST.

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ARRIVING ATL FROM N; PLANNED ILS RWY 26R. APCH ASSIGNED ILS RWY 27R SIDESTEP RWY 27L. REBRIEFED AND SET UP FOR NEW APCH. ATIS WX WAS WINDS 240 DEGS AT 7 KTS; 9 MI; TSTMS; FEW AT 5000 FT; TEMP 34 DEGS; DEWPOINT 20 DEGS; 30.03. TSTMS AROUND FIELD BUT NOT AT FIELD. WE WERE CLRED FOR APCH AND THEN HANDED OVER TO TWR. TWR INFORMED WINDS WERE 140 DEGS AT 18 KTS GUSTING TO 28 KTS; THEN I MIN LATER 150 DEGS 19 KTS GUSTING TO 28 KTS. OUR MENTAL CALCULATION WAS THAT TAILWIND (GUST IS NOT INCLUDED) COMPONENT SHOULD BE ABOUT 1/2. SINCE IT WAS CLOSE TO OUR LIMIT I WANTED TO FIGURE EXACTLY. MY CALCULATION ON THE COMPUTER KEPT COMING UP WITH CLOSER TO 15 (I WAS CALCULATING XWIND INSTEAD OF TAILWIND); AND I BECAME DISTR FROM THE APCH. THE FO STARTED TO CONFIGURE FOR LNDG WITHOUT MY KNOWLEDGE. AS THE ARPT CAME INTO SIGHT; WE WERE CLRED TO LAND ON RWY 27L. AS THE FO MADE TRANSITION; WE RECEIVED A 'GS WARNING' WHICH WAS NORMAL FOR SIDESTEP MANEUVER. WE THEN RECEIVED 'TOO LOW TERRAIN.' REALIZING WE HAD FORGOTTEN TO DO A BEFORE LNDG CHK; I MADE A QUICK ASSESSMENT AND REALIZED WE DID NOT HAVE LNDG FLAPS. I COMMANDED A GA AND WE RETURNED FOR LNDG WITH NO INCIDENT. I ALLOWED MYSELF TO BE DISTR BY THE TAILWIND CALCULATION. WE SHOULD HAVE GONE AROUND AT 1000 FT IF WE WERE NOT SURE OF THE LEGALITY OF THE LNDG. I'M A NEW CAPT -- FELT FORTUNATE TO LEARN LESSON WITHOUT BENDING METAL OR LOSS OF LIFE!