The World Record set with AMD's FX-57 and is right at 4GHz.

The World Record set with Intel's 965 series and is around 7.2GHz.

Both are using liquid nitrogen.

AMD's fastest FX single core chips are very hard to overclock much past 3GHz, and AMD's dual core are hard to reach 3GHz, yet Intel's dual core chips usually can reach near or just over 4GHz only using a good air cooling system.