Just go into your BIOS and raise the FSB until it won't boot into Windows properly.
Then, raise the VCore voltage (basically increase the amount of electricity to the CPU) by one step. This is when you have to be very careful about temperatures though. Watch them carefully, I think the max on that processor is 65*C.
Repeat until you hit a wall. At this point you'll probably have to increase your chipset or RAM voltages. Watch
Once you get as far as you think you can, test for stability using the Prime95 torture test:
Run it for a few hours, if you don't get any errors, then it's probably stable. I don't know how far you'll get with stock cooling, but it's worth a try, especially if your temps are that low already. Although I doubt CS:S is really stressing that processor much...