VISUAL SEPARATION ATTEMPTED BY APCH CTLR WITH TA GIVEN. WRONG TFC SIGHTED BY FLC.

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I WAS PF ON APCH TO SEA. WX WAS 2100 FT SCATTERED; VISIBILITY 10 MI IN HAZE. ON L DOWNWIND ABEAM AT ABOUT 5000 FT; APCH CTLR POINTED OUT A 737 AT 11 O'CLOCK (I DON'T REMEMBER THE RPTED RANGE). WE SPOTTED AN ACFT AT THAT AZIMUTH (INCIDENTLY WINDS WERE EASTERLY ALOFT) AND RPTED THAT WE HAD THE RPTED ACFT IN SIGHT; ATC TOLD US THAT HE WAS FOR RWY 34R; WE WERE CLRED FOR VISUAL RWY 34L. WE TURNED BASE REF THE ACFT WE THOUGHT TO BE OUR TFC. WHILE TURNING BASE; WE SPOTTED ANOTHER ACFT BEHIND THE ONE WE THOUGHT WAS OUR TFC. ATC; SIMULTANEOUSLY POINTED OUT OUR MISTAKE; AS IT TURNED OUT THE SECOND ACFT WAS OUR TRUE TFC. BOTH ACFT WERE FOR RWY 34R; SO WE CONTINUED FOR AN UNEVENTFUL LNDG RWY 34L; ALTHOUGH WE WERE ALIGNED FOR RWY 34L AHEAD OF OUR TFC FOR RWY 34R. I CONTINUED TO VISUALLY REF THIS TFC SLIGHTLY BEHIND AND TO OUR R ON FINAL. WE FOLLOWED THE WRONG ACFT INITIALLY; AND THAT IS OUR FAULT. HOWEVER; THE POINT OUT THAT ATC GAVE US CERTAINLY DESCRIBED THE APPROX POS OF THE ACFT THAT WE MISTAKENLY IDENTED; AN A-320. ATC NEVER POINTED THIS ACFT OUT TO US; A SIMPLE COMMENT THAT WOULD HAVE PREVENTED OUR MISTAKE; IE; 'YOU'LL SEE 2 ACFT ON FINAL; THE FIRST IS A A-320 THE SECOND A B737 YOUR TFC IS THE SECOND.'