Too Much Choice?
Can you differentiate all the Intel brands from one another without looking up the details? Being low-cost products, Celeron D and Celeron M have much more in common than Pentium D and Pentium M.
In terms of its design, Intel's model numbering method certainly is logical and easy enough to understand. The first of the three numbers represents the processor class, with higher numbers meaning a better, more expensive choice. The processor speed is characterized by the second digit, higher numbers represent faster clock speeds. An example would be the Pentium 4 570, which is considerably faster than a Pentium 4 530. However, this rating is only valid within each processor class - comparing processors across platforms is not possible.
Intel considered an extension of the product portfolio to be an advantage for the customer, so they added the suffix 'J' for marking Pentium processors that support the XD bit. Too bad this is not valid throughout the whole portfolio.
Assigning the third digit to be the feature differentiator further increased the numbering granularity. Pentium 4 500 series model numbers ending in 1 represent those with 64 bit capabilities (EM64T enabled, e.g., the Pentium 4 model 561). At this point it's good to know that this does not apply to the 600 series, since all members of that family are already blessed with EM64T. But there is another use for the 1 there, as it marks the latest 65 nm offering of this processor class (e.g., the Pentium 4 651).
There will also be 600 series processors ending on 2, suggesting a more advanced product. However, this is not entirely accurate. While a Pentium 4 672 is going to support Intel's Virtualization Technology (VT), it will be based on the old 90 nm production process, not 65 nm.