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#11
Overclocking the CPU by frequency alone isn't nearly as unsafe as with increasing the voltage. If you look at the formula P = C*F*V^2 where P = Power, C = Capacitance, F = Frequency and V = Voltage. You can see power increases with the square of voltage, yet only linearly with frequency.
Ken summed it up pretty well. There's still a risk, but a small overclock with no voltage increase won't yield as big of a result, but less risk than a larger overclock with a voltage increase.