safely for your hardware is below Max tempereture 70C ~ 80C when fullload under Stress Test , use Software Real Temp / Hw Monitor Pro u can See Max temperature.
if u have over Temperature, U must go down overclock !
Intel's stock cooler does not do much for overclocking.
Prolimatech Megahalem Rev. B for Intel 775 / 1156 / 1366 is a great after-market cooler and will work well for you. When using this (or any other heatsink) check for clearance between the heatsink and the RAM modules.
Depending on the chip and motherboard,you should be able to achieve a 10 to 15 percent overclock as long as you don't raise the voltage and you have good case airflow. Can't hurt to try, many can mildly overclock their chips on stock cooling without heat issues. If your looking at a moderate or high overclock, don't bother with stock cooling and buy a nice after market heatsink.