The overclocked CPU is sure to generate more heat than its more conservatively paced brethren. A heat sink and fan combo are definitely required for any overclocking project. A heat sink is a small set of metal fins, like the surface of a motorcycle engine, that fits onto the component. The conductivity of the metal and the extra surface area transfers the heat from the CPU and allows it to dissipate. Heat sink compound must be applied directly to the chip so that no air space is between the processor and the heat sink.
Air is a very good insulator.
An air gap will hinder the transmission of heat from the processor to the heat sink. The heat sink fans do wear out. This is particularly true in tower case systems for here the fan has to work against gravity. At any rate, DO peek inside from time to time to make sure that all is well. To really insure against anxiety and sleepless nights, get one of the special thermostats with an alarm to watch the temperature.