WHEN THE PLT CALLED THE FSS AT 1341 MST, HE STATED HE HAD ALREADY RECEIVED SOME 'REAL TIME' WX & REQUESTED ONLY THE WX AT DURANGO & FARMINGTON, NM. HE FILED AN IFR FLT PLAN TO DURANGO & TOOK OFF AT 1437. DRG ARRIVAL, THE PLT ATTEMPTED TO GET INTO VFR CONDITIONS TO LAND AT PAGOSA SPRINGS, ABOUT 30 MI ENE OF DURANGO. HOWEVER, HE WAS UNABLE, SO HE CONTD TO THE FILED DESTINATION. AN ILS/DME RWY 2 APCH WAS MADE TO CIRCLE & LAND ON RWY 20 AT DURANGO. AT 1925:44, THE PLT RADIOED THAT HE HAD THE ARPT IN SIGHT. THE AIRCREW OF SUNWEST FLT 204 SAW N6400E BREAK OUT IN THE CLEAR & ENTER A LEFT DOWNWIND FOR RWY 20. HOWEVER, THEY LOST SIGHT OF N6400E AS IT WAS ABEAM OF THE APCH END OF RWY 20. APRX 5 SEC LATER, THECOPLT OF FLT 204 SAW THE LIGHTS OF N6400E BREAK OUT OF THE CLOUDS, & APRX 1 SEC LATER, HE SAW 'A LARGE GREEN BALL OF FIRE.' AN EXAM OF THE CRASH SITE REVEALED THE ACFT STRUCK A POWER LINE ABOUT 30 TO 35 FT AGL WHILE CONFIGURED FOR LANDING, THEN CRASHED. THE ELEV OF THE UNCONTROLLED ARPT WAS 6685 FT, THE CIRCLING MDA WAS 7100 FT. WND WAS GSTG 12 KTS.