It's been a long time since MHz were the only part of a CPU's specification that concerned people. Manufacturers take different approaches to CPU design, even when using the same architecture. That means for example, at the same clock speed, an Intel and AMD processor will deliver different performance.
AMD were the first to move away from selling processors based on clock speed, instead opting for a rating system that attempted to liken the performance of one of their processors to a differently clocked Intel processor. However, recently Intel also moved away from using clock speeds as their main marketing tool, introducing a new set of model numbers.
This article should help you to understand how to interpret product codes and what to look out for when trying to find a new product.
