A B737-800 PILOT ACCEPTED A RWY CHANGE FOR A VIS APCH AND WHILE PROGRAMMING THE FMS FOR THE NEW RWY GOT HIGH AND EXECUTED A GAR.

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ACR FLT. FLYING THE RNAV ARR. APCH CTL OFFERED A VISUAL APCH. I ACCEPTED THE VISUAL APCH SINCE THE ARPT WAS IN SIGHT WITH GOOD VISIBILITY AND NO TFC AHEAD OF US WITHIN 5 MI. I HAD PROGRAMMED THE RNAV GPS RWY XX INTO THE FMS PRIOR TO DSCNT EXPECTING THIS RWY. WHEN GIVEN THE VISUAL APCH; I EXTENDED THE FAF FOR RWY XX IN THE FMS AS A REF FOR THE RWY EXTENDED CTRLINE AND FOR A REF ALT. I SPENT TOO MUCH TIME HEADS DOWN IN THE COCKPIT AND WAS HIGH ON L BASE TO PERFORM A NORMAL DSCNT AND LNDG. FO RELAYED TO THE TWR WE WERE HIGH FOR THE LNDG. TWR TOLD US TO TURN L TO THE SE OVER THE FIELD AND CLB TO 7000 FT. TWR THEN PASSED US BACK TO APCH FOR VECTORS FOR A L DOWNWIND ILS RWY YY. IT IS NICE TO HAVE ALL THE 'MAGIC' WITH THE FMS; BUT SPENDING TOO MUCH TIME HEADS DOWN IN THE COCKPIT KEPT ME HIGHER THAN I WANTED TO BE FOR THE VISUAL APCH. TO PREVENT THIS SIT FROM OCCURRING AGAIN; I'LL SPEND LESS TIME HEADS DOWN; HAVE THE FO MAKE ANY CHANGES IN THE FMS; AND MOST IMPORTANTLY JUST FLY THE ACFT VISUALLY TO THE ARPT USING OUTSIDE VISUAL REFS.