Intel Overclocking Terminology
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Base Clock (BCLCK): The Core i7 processors saw the retirement of the Front Side Bus utilized by the older Intel processors. Base Clock is its replacement. Base Clock is essentially the reference clock for many sub-components of the processor, including the memory, core speed, L3 cache, and QPI. This is one of the most important tools available for overclocking the processor. Keep in mind, however, that increasing this setting does affect components other than your core speed, which can lead to instability.
Turbo Mode: Some of the newest intel Processors offer this ability. In essence, it is an automatic overclock for a single core of the processor, if the task at hand would benefit. The benefit gained can vary dramatically, since this only affects one of the cores on the chip. For single threaded apps, this is a nice benefit - but for multi threaded apps, the benefit will essentially be zero. If your motherboard supports changing this setting, it may be worthwhile to test different scenerios with this enabled and disabled, to see which benefits your particular situation the most.
Core: The main processing portion of the chip is called the "core". This is the area directly involved in raw calculation and execution, and is the part you're generally looking to overclock. Changing this speed will have the most dramatic effect on performance.
Uncore: This portion of the chip is basically everything that is not part of the "core". This includes the L3 cache, memory controller, and Quick Path Interconnect. You will generally see minimal gains from overclocking this portion of the chip, with the side effect of added instability in doing so. As such, your best practice is to use settings which maximize your Core overclock while keeping the Uncore clock as close to stock as possible.
Mutliplier / Ratio: The core speed of the processor is determined by multiplying the Base Clock by this number. On most processors, this number is "upward locked" preventing users from increasing it beyond the factory default. Most chips, however, are unlocked downwards, allowing you to run a lower multiplier if you so choose. This may be desirable to reduce power consumption, or if your circumstances mean running a higher Base Clock with alower Mutliplier is advantageous. There are "Extreme" chips available, which are upward unlocked, allowing you to both increase and decrease this value, but they are significantly more expensive than the standard locked product offerings.
SpeedStep / C1E: This automatically reduces the CPU Multiplier when the processor is idle. This feature saves energy, and reduces heat output when the chip is not being fully utilized. In some cases, disabling these settings can lead to a more stable overclock.
Load Line Calibration (LLC): When a CPU goes from idle state to running with a load, there is generally a drop in voltage supplied to the processor. Enabling this feature will make the motherboard try and supply a steady amount of voltage the entire time. It is generally a good idea to keep this feature enabled.
QPI Frequency: This is the base speed of the inteconnect between various components. You will see little to no benefit in increasing this speed. When increasing your Base Clock, this value will increase accordingly. It's recommended that you reduce the QPI multiplier to try and keep this speed as close to the stock value as possible.
CPU Spread Spectrum: Enabling this setting will try to reduce some of the electromagnetic interference caused by the pulses from the CPU clock. However, it can also reduce stability.
CPU Voltage: This is the voltage supplied to the processor. Generally speaking, higher voltages produce a more stable overclock, but also significantly increases heat output and can reduce the life of the chip. Conversely, lower voltages will reduce heat output and possibly extend the life of the chip, while potentially reducing stability. The optimum goal is to run the lowest possible voltage for a given overclock.
PCI Express Frequency: Increasing this value can help the data throughput from your graphics card. It does, however, offer yet another option for system instability. Most users will wish to keep this static at 100Mhz, but you may increase it and tweak as desired.
DRAM Frequency: Increasing this value will increase the bandwidth available to your memory. This can increase system performance, but only if the memory is capable of handling the additional speed.
PCH Voltage: This is the voltage applied to the chipset itself, and should not require any real tweaking.
VTT Voltage: This it the voltage applied to the QPI. Increasing this can increase stability, but do so with caution, as it can also significantly increase heat output and reduce the life of your motherboard.
DRAM Voltage: This is the voltage applied to the memory. You must keep a maximum of a .5v difference between the Core voltage and the DRAM voltage. As with other voltages, increasing this can increase system stability, but can also destroy your RAM and increase heat output. Adjust with caution.