You cannot determine the amount that you will be able to overclock as it completely depends on the current hardware configuration, enviromental conditions, methods of cooling, etc.
With the current temperature of the CPU, you can expect to see a decrease in the life span. The temperature is not hot enough to damage it.
To lower the temperature of the CPU, I would recommend changing the current heatsink/fan and thermal compound.
For the heatsink - Go with alluminum fins, but a thick copper plate for the base. This combination makes a very effective heatsink as copper is most efficient at absorbing heat while alluminum is most efficient at releaving heat.
I couldn't recommend anything else but Arctic Silver 5 for the compound. There are a lot of compounds out there that appear silver, but do not have any present in the ingredience. So don't be fooled.
P.S.: Keep in mind that you should have more exhaust fans than intake.