Depending on the exact CPU model, the maximum rated CPU die temperature is either 85 or 90 degrees C.
There is an internal thermal diode that will shut down the system if the CPU temperature exceeds 125 degrees C.
As to cooling, first make sure that there is enough free airflow in your case.
I add case inlet fans to my systems.
Per AMDs thermal guidelines, you want cool air coming in over the heatsink fan and the hot air from the heatsink should be sucked away through a bottom fan on your power supply.
If your power supply does not have air vents or a fan on the bottom, changing to a better power supply with the fan may be all you need to do.
Once you have enough cool air flow into the case, you might want to look into some of the copper finned heatsinks.
Some of these even use external fans.
The thinner copper fins allow for more thermal dissipation than a normal aluminum heat sink.
I like fans from Thermaltake and Thermal Integration Technologies my self.
A source for copper heatsinks is www.cyberguys.com
Chas