Overclocking the Core i7 was a breeze. I simply entered the BIOS on the ASRock P55 motherboard that I was using and set the BCLK to 202, adjusted the voltage and RAM divider and I was off and running at a very nice 4030MHz completely stable. At that speed, I was able to run all of our tests back to back without any issue and even found myself running a little over on the gaming sessions.
For the Core i5 we did pretty much the same thing except I pushed the multiplier to 21. This netted us a very nice 4.2GHz overclock on the Core i5 750 - roughly a 59% overclock and not bad at all for taking only three minutes to get going.
You can see the validation for the i7 870here, while the i5 750 is here.
I am confident that with better cooling and some tweaking, you would be able to get much higher with both of these CPUs.
As all overclocking results are dependent on the hardware you use your results may vary. Results of our overclocking tests are included in the performance section with the stock scores.
Important Editor Note: Our maximum overclocking result is the best result we managed in our limited time of testing the motherboard. Due to time constraints we weren't able to tweak the motherboard to the absolute maximum and find the highest possible FSB, as this could take days to find properly. We do however spend at least a few hours overclocking every motherboard to try and find the highest possible overclock in that time frame. You may or may not be able to overclock higher if you spend more time tweaking or as new BIOS updates are released. "Burn-in" time might also come into play if you believe in that.