"On the I7 965 Extreme the processor is unlocked so the multiplier can be increased substantially to increase the CPU speed without touching the base clock frequency while the I7 920 clock multiplier is locked at 20 so to really push the CPU you will need to push the base clock frequency to see an increase in performance. On the 965 I tried both increasing the base clock multiplier as well as the baseclock frequency to end up with a final stable speed of 3.970 GHz for my benchmark testing. This was accomplished by setting the base clock frequency to 147MHz with a clock multiplier of 27. Rather than using the voltage offset in the BIOS I set the voltage manually. To keep the CPU from throttling under load you can increase the "current limit" override as well as the "power limit" override in the DX58 Smackover motherboard BIOS. To increase the base clock frequency you can only go so far until you will need to add a little voltage to the IOH core voltage override and QPI voltage override in the bus override section of the BIOS. It's just a matter of learning the ins and outs of the BIOS and what the processor likes, 3.970GHZ and 3.100GHZ are what the two processors liked on air without cooking parts. Of course, no two chips are alike so your mileage may vary."
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Original URL, circa 2008: http://www.overclockersclub.com/reviews/intel_core_i7/