Overclocking the older Pentium 4 series of processors, especially Prescott, Presler and Ceder Mill based cores was a timely experience resulting in a huge power draw off the PSU and extreme amounts of heat being generated by the processor - either loud air cooling solutions or expensive water cooling rigs were required to keep them operating properly.
Core 2 is simply the answer to all our processor prayers. Not only does Core 2 show a good overclocking curve, even from these early samples we have, but the amount of heat generated per MHz over its default is less than half of what the Pentium 4 generated. Adding to that the reduction in the amount of wattage the processor needs to pull from the PSU to do its job results in a quieter and cooler system.
Core 2 Duo definitely is the more affordable option in the processor realm from Intel and presents good overclocking value. If you want full control over your overclocking which will at the end of the day result in a higher overclock, Core 2 Extreme with its unlocked multipliers, higher starting clocks and higher overclocking ability makes more sense as you can go for a higher bus speed with memory running in a 1:1 ratio to give an overall system boost or simply go for the MHz race and get as much out of the processor to be the king of the hill.