If you are going to use the stock heatsink, you will not be able to increase the voltage much. So go into the BIOS and start increasing the FSB freq 10 MHz at a time, then testing for stability and temperature. You will be able to do this 3, maybe 4 times, before stress testing fails. Check your temperatures. If your temperatures are below about 70 C, increase the vcore voltage about .05 volts.
Keep working back and forth between FSB freq and vcore while keeping your load temps below 70 C until you have gone as far as you can.