Cool n' Quiet is an AMD feature that allows you to save electricity when the computer is idle. When the CPU is idle and Cool n' Quiet is enabled the motherboard halves the CPU clock multiplier and throttles down the voltage. With less voltage to the CPU it runs cooler. Since the CPU runs cooler the CPU fan gets throttled down to, thus, making the system quieter. When you begin using the computer again the CPU multiplier automatically and voltage ramps back up to stock speed.
It is possible to overclock while running Cool n' Quiet and people do it all the time. The caveat is that you need to make sure you leave your multiplier at stock and overclock by increasing the HTT (FSB). Cn'Q works by lowering the multiplier to around 5x and lowering the CPU Voltage. This makes the processor run slower and cooler. Then the motherboard will sense that it's cooler and lower the CPU fan speed, thus making your system quieter.
If you overclock by lowering the CPU multiplier and then raising the HTT you'll get instability. When Cn'Q needs to restore your multiplier when the CPU is under load, it will use the stock multiplier no matter what you have it set to in BIOS. That could result in an overclock that's higher than your board supports at your settings and you'll get lock ups. So as long as you overclock with your processor's stock (maximum) multiplier you'll be fine with Cn'Q.