After departure from Sacramento, the pilot of the C-210, N784RF, had declined further radar traffic advisories, and was climbing on about a 152 deg heading. The pilot had changed radio frequency, and was receiving weather advisories for his destination airport. The C-180, N3313D, was in cruise flight at 3,100 ft on about a 307 deg heading, and the pilot had just terminated his participation in receiving radar traffic advisories from the Stockton TRACON. The two airplanes collided near head-on about half way between two TRACON facilities. The right wing of the C-210 contacted the left wing of the C-180, and both wings separated about mid-span in about a 90-deg orientation from each other. Visibility was reported as 10 mi in haze. Other pilots reported that smoke limited visibility from 1 to 5 mi in the area. Both airplanes had Mode C altitude encoding, and both were depicted on radar prior to the collision.