RAM Speed

I have heard a few people say that buying faster RAM will not necessarily increase your computer performance. For example, this guy on PCGuide.com said:

Socket AM2 AMD systems are better off with DDR2-800 while Core2 Duo systems will not use more than DDR2-667.

Is this true? And if it’s true, why do computer sellers (HP and Dell, for example) let you configure your system with overclocked 1066 Mhz RAM when doing so will not be be fruitful?