the E8400 is know to have buggy/faulty temperature sensors. My CPU idles at 45C and generally does not read a temperature above 45C (I got it to once using a stress test with an Intel temp sensor, but that was it). I don;t have it OCed, but still there should be some fluctuation in the temp.
Based on my knowledge of CPUs, I would imagine that the true idle temp would be significantly lower then this (I would estimate it is actually idling in the high 20's to low 30C range), but the temp sensor is "stuck"
Based on these known issues I would take any temp readings with a grain of salt.