If you have a good copper heatsink and don't OC, heat won't be a problem. I think that below 60C is safe but you should know that the cooler a CPU (to a dagree) the faster it will run, and I'm not talking about overclocking, so try to keep it below 50.
Modern CPUs have a feature that slows them down (by underclocking or other means) whenever the temperture exceeds a threshold, and that hurts performance badly.
My anger and my wrath will be poured on this place on man and beast on the trees of the field and the fruit of the ground it will burn and not be quenched Jeremiah 7:20
I am ALPHA and OMEGA, burn!