The numbers game: Intel vs AMD
In marketing their respective CPUs, both Intel and AMD play games that can be confusing to the first-time CPU buyer. Both use numbering schemes which are built around the same central premise, namely that describing a processor in terms only of its clock speed is misleading in that it doesn't paint a complete picture of the processor's performance when you take into consideration its other features.
AMD was the first to essentially drop processor speeds from its CPU descriptors. It now gives all of its processors model numbers that are intended to indicate relative software performance within the AMD family of processors. So an AMD Athlon 64 X2 5000+, for example, isn't a processor with a 5GHz clock speed -- it's actually running at 2.6GHz, but in AMD's ordering of things, that's where it sits.
The exception to this rule is the Athlon 64 FX series, which simply have two digit oddly sequenced numbers to differentiate them. The first Athlon 64 FX chip was the FX-51, with the most recent being the FX-74. According to AMD, the lack of a relative model number within the FX series is because the higher-end customers for Athlon 64 FX CPUs are more likely to benchmark the processors themselves in order to sort out any performance confusion issues.
Intel's take on processor numbering removes the core clock speed from the equation altogether. Instead, what you get is a four-digit number, which is preceded by one or two letters, as well as a model name that designates the capabilities of the processor, be it dual-core or quad-core, along with some indication of where it stands relative to other processors within the same processor family. Number differences don't always represent speed ratings, but can indicate the presence or absence of other features such as if the CPU has a faster front side bus speed.
As an example, the Intel Core 2 Duo E6700 and the E6750 differ only in their front side bus speeds (1066MHz, as opposed to 1333MHz). Meanwhile, the Core 2 Quad Q6600 and the Core 2 Duo E6600 have the same clock speed (2.4GHz), despite the Core 2 Quad Q6600 being a quad-core CPU, and the Core 2 Duo E6600 a dual-core CPU.