ACR LGT WRONG RWY APCH GO AROUND.

Narrative

ON DOWNWIND; WE WERE ADVISED OF SHORT TURN ON FOR VISUAL TO 17L. BASE TURN WAS MADE ABOUT 1 MI OUTSIDE MARKER. ON BASE; OUR RWY WAS CHANGED TO 18R. CAPT RETUNED ILS AND FMS SET UP. I SET ACFT UP TO TURN 'ON TOP' MARKER FOR RWY. CAPT SAID TURN INSIDE MARKER OK. IF I WANTED TO. HDG SELECT USED TO TURN INSIDE MARKER REFERRING TO HSI DISPLAY. DURING TURN; WHILE CHKING INSIDE; ACFT APPARENTLY TURNED FARTHER THAN EXPECTED AND WHEN I LOOKED OUT; I 'LOCKED IN' ON 17R. TWR CALLED TO CONFIRM OUR LINE UP FOR 18R; THEN CLRED A VISUAL TO 18L; IF POSSIBLE. AT SAME TIME; WE REALIZED OUR LINE UP ERROR. CAPT MISUNDERSTOOD TWR TO SAY 17L APCH OK. TWR SAID NEGATIVE. 18L VISUAL OK. BY THEN; IT WAS TOO LATE (500' AGL) TO CHANGE RWYS AND GO AROUND WAS EXECUTED. THE MISSED LINE UP WAS CAUSED BY INATTENTION TO BACKUP INSTRUMENTS AND ACFT TURNING FARTHER THAN EXPECTED; WHILE MY HEAD WAS IN COCKPIT. ALSO; CONSTANT RWY CHANGES AT DFW ARE COMMON AND CONFUSING; I.E.; 17L; 17R; 18L; 18R; ETC. ATLANTA RUNS A MUCH BETTER SET OF 4 PARALLELS. I SEE NO NEED FOR DFW'S ERRATIC RWY CHANGES.