After crossing The Dalles at 12,500 ft MSL, the pilot reported to ARTCC he was entering solid clouds. He subsequently reported level at 3,000 ft MSL followed by 'if ah we can't stay VFR we'll go ahead and climb up to about five and file IFR.' The aircraft was observed flying in & out of patches of 'light fog' at about 450 to 500 ft AGL near an interstate highway & close to mountains. A witness reported the top third of the mountains adjacent to the gorge was obscured. The aircraft impacted a steep sloping ridge. On previous portions of the cross-country flight, which originated in Alabama, the pilot had flown in IFR conditions & had executed an instrument approach. His logbook indicated that he had flown a total of 16.6 hours of actual instrument time. The aircraft's CG was estimated to be approximately .6 inch behind the aft limit. Two of the occupants were small children weighing 31 & 37 lbs.