First,you won't get a noticeable boost in games that don't benefit from more than 2 cores.
As for OC'ng,no one can exactly tell you how much you can OC your CPU.
Here are some reviews:http://hothardware.com/Articles/Intel-Core-2-Quad-Q8400S1/
"We love free upgrades. One of the best ways to get more performance from your PC is to overclock the processor. From a stock frequency of 2.66GHz, we overclocked the Q8400 to an impressive 3.92GHz using a TRUE 120 and 1.36V. We started with a 333MHz FSB and raised it slowly while testing for stability along the way. It wasn't until we passed 490MHz FSB that we ran into any issues so we backed it back down and settled for the 3.92GHz overclock. We would like to note that we stayed within Intel's recommended operating voltage (0.85V - 1.3625V) to achieve this overclock. With higher voltages and more powerful cooling, even higher frequencies are likely possible."http://www.anandtech.com/show/2762/12
"The Core 2 Quad Q8400 on the other hand is based on a core that's been shipping for quite a while, at stock voltage using the stock cooler I was able to hit 3.08GHz"