DURING THE PILOT'S INITIAL WX BRIEFING AT 1101, HE WAS ADVISED VFR FLIGHT NOT RECOMMENDED THROUGH MOUNTAINS UNTIL COLD FRONT HAD PASSED. DURING A LATER 1331 WX BRIEFING HE WAS INFORMED THAT WX CONDITIONS WERE DETERIORATING SOUTH OF ROANOKE, VA, THROUGH HIS DESTINATION AIRPORT. THERE WERE FLT PRECAUTIONS FOR LIGHT-MODERATE RIME ICING FROM FREEZING LEVEL OF 8,500 FT TO 12,500 FT. LATER AN UNIDENTIFIED ACFT WAS OBSERVED ON RADAR CIRCLING AT 8,500 FT, THEN DESCEND TO 5,500 FT, THEN HEAD SOUTHWESTBOUND NEXT TO THE FINAL APPROACH COURSE FOR ILS RWY 23 AT TRI-CITY REGIONAL AIRPORT. IT THEN TURNED WESTBOUND AT 1810 BEFORE DISAPPEARING FROM RADAR. THE AIRPLANE WAS OBSERVED DESCENDING ALONG A RIDGELINE; GROUND FOG TOTALLY OBSCURED THE TOPS. THE AIRCRAFT IMPACTED TERRAIN AT ABOUT A 90-DEGREE NOSE-DOWN ATTITUDE. PILOT'S TOTAL RECORDED NIGHT TIME WAS 5.5 HRS; LAST FLIGHT 1/89. LAST BIENNIAL 29 MONTHS BEFORE ACCIDENT. PILOT HAD PREVIOUSLY SPOKE OF 'PUSHING THE LIMITS' IN REGARD TO FLYING IN MARGINAL WX CONDITIONS.