The HardwareSecrets gang goes above & beyond in their efforts to o/c an S775 P4 setup. They even posted a handy how-to guide for you to check out if you're so inclined to follow in their footsteps.

"The first difficulty is that since the first Pentium 4 the clock multiplier is locked. Only engineering samples (that are made for testing) and the Extreme Edition models have the unlocked clock multiplier. Engineering samples are identified with the letters “ES” and are not available in the market."