BEFORE DEPG BLAIRSVILLE, THE PLT REQD THE CURRENT ATLANTA WX FOR THE DEKALB-PEACHTREE ARPT & FOR AN AFTERNOON OUTLOOK. HE TOLD THE BRIEFER THAT HE HAD RECEIVED A PREVIOUS WX BRIEFING. HE WAS INFORMED THAT THE FORECAST WAS FOR INSTRUMENT CONDS UNTIL NOON WITH A CHANCE OF OVERCAST SKIES AT 1500 FT. HE DEPARTED BLAIRSVILLE AT 1210 EST WITH INTENTIONS OF FOLLOWING HIGHWAYS TO THE ATLANTA AREA. A PERSON, WHO WAS DRIVING A CAR BETWEEN BLAIRSVILLE & THE SUBSEQUENT CRASH SITE, SAW A MOONEY FLYING IN & OUT OF FOG AT ABOUT 150' AGL. THERE WAS RAIN, LOW CEILING & POOR VIS ALONG THAT ROUTE. WHEN THE ACFT DID NOT ARRIVE AT THE DESTN, A SEARCH WAS BEGUN. THE ACFT WAS FND WHERE IT HAD CRASHED ON RISING, WOODED TERRAIN ON MT OGLETHORPE AT AN ELEV OF ABOUT 3290'. AN EXAM REVEALED THE ACFT WAS IN LEVEL FLT JUST BEFORE IMPACT. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FOUND. ALTHOUGH THE PLT WAS INSTRUMENT RATED, HE HAD NO RECENT INSTRUMENT FLT TIME.