B737 CREW LNDG SEA OVERTOOK A DH8A WHILE CONDUCTING A VISUAL SIDE BY APCH TO THE ARPT.

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WHILE BEING VECTORED FOR A VISUAL APCH INITIALLY TO SEA RWY 34L WE WERE SEQUENCED BTWN A NUMBER OF ACFT ARRIVING SEA FROM THE SE AND SW. WE WERE ASKED TO RPT SIGHTING A DASH-8 OVER SEATAC MALL AS WE WERE ON A BASE LEG ARRIVING FROM THE E. AFTER RPTING THAT ACFT IN SIGHT; WE WERE CLRED FOR A VISUAL APCH TO RWY 34R AND TO MAINTAIN VISUAL SEPARATION FROM THE DASH-8. WE HAD SLOWED TO 170 KTS AND WERE APPROX 3 MI OR MORE BEHIND AND TO THE R OF THE DASH-8. I DO NOT RECALL ANY INSTRUCTIONS TO THE DASH-8 TO MAINTAIN A FASTER APCH SPD. AS WE CONTINUED TO CONFIGURE AND SLOW (OUR ADJUSTED REF SPD WAS APPROX 150 KTS) FOR A B737-900 LNDG AT APPROX 141000 LBS; OUR CLOSURE RATE ON THE DASH-8 RAPIDLY INCREASED. TWR RECOGNIZED THIS AND POINTED US OUT TO THE DASH-8 BEFORE WE PASSED HIM. WE EVENTUALLY WENT BY THE DASH-8 AT OUR ADJUSTED REF SPD. IT WAS NIGHTTIME AND VISUAL CLOSURE RATES AT NIGHT ARE DIFFICULT AT BEST TO JUDGE. WE HAD ADJUSTED OUR FINAL APCH COURSE INTERCEPT HEADING TO REDUCE THE ANGLE OFFSET INITIALLY GIVEN BY APCH CTL IN ORDER TO AVOID ANY CHANCE OF A COURSE OVERSHOOT. MY RECOMMENDATION IS NOT TO ACCEPT VISUAL SEPARATION CLRNCS AT NIGHT WITH LESS THAN 5 MI INITIAL SEPARATION WHEN FOLLOWING A TURBOPROP.