I wouldn't go anywhere further than 3.6-3.8GHz with that chip. The P5K IIRC has only one 4pin CPU connector, which cannot supply enough power to the PWM. Overclocking your quad on that board may cause the connector to melt, literally. But that aside, if you really want to go further, you'll have to find the bottleneck. This means setting your CPU multiplier a bit lower, then increasing the multiplier of your RAM. Increase in increments and test for stability until the RAM finally gives in. Repeat for your mobo's HT Link/FSB. Then raise your CPU's multi to max and adjust frequency accordingly.