From "Official ASUS P8P67 Series Overclocking Guide and Information" by Juan_Jose, ASUS Technical Marketing Specialist (http://hardforum.com/showthread.php?t=1578110)

1. For K series parts, the stock voltage supplied will allow for consistent overclocking generally up to a multiplier of 43x. There is potential for the multi to be raised to 44x depending on the load induced. This default voltage range be approx 1.240 to 1.260 under load.
2. Increased range between 44 to 47x multipliers will generally require a voltage range between 1.30 to 1.375V with an LLC recommended setting of high to ultra high.
3. Increasing the range between 48 to 50x multiplier will generally require a voltage range between 1.40 to 1.500 with a LLC recommended setting of ultra high.
4. Increased range between 50 to 52 (52 generally considered peak max multiplier except for rare 54x parts) will generally require a CPU voltage range between 1.515 to 1.535V with LLC at Ultra High and potential fine adjustments to the CPU skew range.