It's best to leave the cool and quiet alone unless you keep your comp and stock and have a cold room or something. It automatically adjusts the fan speed to your temperature, and with us overclockers it isn't much good, cause it'll be running high most of the time. If it's a stock comp in a cool room then it will slow the fan speed making it quiet, hence cool'n quiet.
Cool 'n' Quiet dynamicly adjusts the system multiplier and voltages based on system load to keep temperatures and power consumption low. Generally it's useful, even if you are overclocking cause it'll drop the system clocks if you're not using it, I actually had to enabled to access some overclocking options on my board.
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