It would be very hard to kill the CPU with heat alone, as the computer would shut off automatically before it got anywhere near hot enough to kill it. Usually, you want to stay under 60c, but if you go a little over it's not going to hurt anything. Now if your CPU gets to 60c on 24/7 settings then you have a problem and you need better cooling. But if it gets that hot with a very stressfull program like Prime95 or S&M, it's not really bad. Voltage is the real killer, not heat.