I overclocked my 6400 to 3.2ghz
and now my cpu is running around 50-60C, is that too high ???
and when my computer is idle, cpu usage is remain 50%, one core is 100% usuage and the other one is 5%, is that normal ???
check on your task manger when you are saying one core is showing 100%, that is not normal , unless you have a problem with one of your software where its hogging all of the CPU's power.
The temps ,well the best way to find if your temps are aceptable is by feeling the HSF on load , and if its touch hot you are OK but if its scolding hot then you are in trouble. If it burns your fingers it will burn the processor but that said the proccessor has more tolerance than your fingers.
To remain completely safe, stay at or below 55C to cater for bad/wrong readings.
One overclock I saw was running at 70C load but I'm not sure if that was luck and if the chip would last.
I have seen a few reports of motherboards under reporting temps.
You should be safe with the temps you are getting.