You will need to keep pushing it higher and higher in small increments until it is no longer stable. That is the only way to know for certain.
i have just overclocked it to 3.2ghz on stock cooler
core #0 is at 51 degrees right now is that normal?
core 1 core 2 and core 3 are 50-47-43 degrees
everything seems to work normal games are a little bit faster too