The stock cooler is seriously crap if you plan on overclocking, even by a small amount. Anyhow, what stepping do you have? I think G0s can hit 2.8 with relative ease.
As soon as you start overclocking, keep an eye out on the CPU temperature, especially since you're cooling isn't upto the task to handle a large amount of additional heat generated. After each bump in speed, run prime95 or wprime to stress the CPU and watch your temperature. If Prime95 runs for a while with everything else within limits, go for the next bump in speeds and repeat the procedure.
P.S.: I hope you've applied some Tuniq TX2 or AS5 for better heat conductivity.