Intel Core i7 CPUs:
We have two CPUs, the 920 and 965XE. The biggest difference besides the free overclockable Turbo-Mode of the 965XE is the L3-cache multiplicator. While the 965XE has a multiplicator of 20, resulting in 2.66GHz, the 920 has only 16, resulting in 2.13GHz, quite a huge difference. The BIOS let us change the multiplicator for the 965XE but didn't with the 920. So, when overclocking consider you can't lower the L3 cache multiplicator.
Both CPU-z screenshots were taken with Turbo-Mode disabled. Turbo Mode with a 965XE will result in 3.46GHz under load. CPUz reports slightly reduced numbers when the CPU is idling but energy savings are turned off:
When idling the multiplicator is reduced to 12: