WHILE FLYING AT NGT IN THE LCL PRACTICE AREA SOUTH OF NAMPA, THE PLT THOUGHT HE HAD FLOWN INTO IMC, BUT THE WX WAS GOOD.HE WAS UNABLE TO SEE THE GROUND, SO HE MADE A 180 DEG TURN, THEN NOTED PROBLEMS SEEING HIS INSTRUMENT PANEL. SUSPECTING PSBL CARBON MONOXIDE, HE TURNED OFF THE HEAT & OPENED THE AIR VENTS, BUT HE THEN EXPERIENCE TUNNEL VISION & BECAME LOST & DISORIENTED. HE PLACED HIS HEAD SET NEAR THE MAG COMPASS WHICH CAUSED AN ERRONEOUS READING. WHILE SEEKING HELP FROM ATC, THE PLT SET HIS HDG INDCR (DGI) TO AGREE WITH HIS MAG COMPASS. ATC TRIED TO ASSIST BY GIVING A DF STEER & PROVIDINGVECTORS DURING A CLIMB TO OBTAIN RADAR CONTACT. HOWEVER, THE ACFT IMPACTED MOUNTAINOUS TERRAIN AT AN ALT OF APRX 4800'. ATC HAD PROVIDED A CORRECT STEER TWD THE ARPT, BUT THE ACFT CRASHED WHILE TRAVELING AWAY FROM THE ARPT. RESCUE WAS DELAYED DUE TO WX & THE PLT TURNING OFF THE ELT FOR A TIME. ATC TAPES VERIFIED NORMAL CONVERSATION UNTIL THE PLT HAD SOUNDS OF PHYSICAL IMPAIRMENT WITH HVY BREATHING, SLOW SPEECH, & DIFFICULTY FOLLOWING ATC INSTRUCTIONS; REASON UNKNOWN.