FLC TUNES WRONG FREQ DURING APCH.

Narrative

PASSING THROUGH 10000 FT MSL; THE APCH CTLR GAVE US A VECTOR TO JOIN THE LOC TO RWY 10 AT MSY. ALTHOUGH THE AUTOPLT WAS ON; THE B737-200 AUTOPLT IS VERY PRIMITIVE AND REQUIRES A GREAT DEAL OF MONITORING COMPARED TO THE B737-300 AUTOPLT WHICH THE PLTS AT MY COMPANY ARE DUAL QUALIFIED IN. IT WAS ALSO A LITTLE BIT TURBULENT AND SOMETHING ELSE REQUIRED MY ATTN AT THAT TIME (I DO NOT RECALL WHAT IT WAS). BUT DUE TO MY WORKLOAD; I ASKED THE FO TO TUNE MY RADIO FOR THE LOC. THE LOC FREQ IS 109.9 BUT THE STANDBY SIDE OF MY NAV RADIO HAD 109.8 WHICH IS A VOR FREQ THAT IS PART OF THE ARR. THE FO MISTAKENLY THOUGHT THAT IT WAS THE LOC FREQ AND FLIPPED THE SWITCH WITHOUT CONSULTING HIS CHART. SINCE THE WX WAS VFR BELOW THE CLOUDS (WE WERE ON TOP OF A BROKEN LAYER) I GUESS HE DID NOT FEEL HE NEEDED TO IDENT IT VIA MORSE CODE (I RARELY SEE PLTS IDENT THEIR NAVAIDS AT ANY COMPANY). I DIDN'T REALIZE QUICKLY ENOUGH WHAT HAPPENED; AND AS A RESULT WE DEVIATED 6 MI S OF COURSE BEFORE ATC ASKED WHERE WE WERE GOING. WE WERE GIVEN A VECTOR BACK TO THE LOC AND LANDED WITHOUT EVENT OR CONFLICT. CLRLY THOUGH; THIS COULD HAVE HAD SERIOUS CONSEQUENCES AT A DIFFERENT LOCATION. THE FO AND I BOTH ARE EXPERIENCED MOSTLY IN EFIS ACFT. HAD WE BEEN IN A B737-300 THIS INCIDENT WOULD NEVER HAVE HAPPENED. MAYBE THAT IS PART OF WHAT MADE US BOTH LESS COMPLACENT ABOUT PROPERLY TUNING THE LOC.