Let's start with Intel's products. When they moved to the new LGA775 style processors, they went from using a product name and processor speed, to using a product name and processor number. The product numbers are three digits long, the first digit tends to signify part of a series. These series currently work as follows:
800 – Pentium D or Pentium Extreme Edition
700 – Pentium M
600 – Pentium 4
500 – Pentium 4
300 – Celeron D or Celeron M
You'll see that without introducing any more numbers, things are a little confusing already. A 300 chip could be a desktop Celeron D or a mobile Celeron M and there are both 500 and 600 range Pentium 4s. The 800s could be Pentium D or Pentium Extreme Edition CPUs.
It's easier to lay out all this information in a table so that you can see the differences in model numbers and also find the (sometimes odd) patterns that emerge.
Pentium D & Extreme Edition
Starting at the top of current performance ladder from Intel, we have both the Pentium Extreme Edition and Pentium D sharing the '840' model number. Both run at the same clock speed, are dual core, have the same FSB and the same amount of L2 cache. However, the Pentium D has enhanced SpeedStep technology to keep it cooler when not under full load, and the Extreme Edition CPU possesses HT (Hyper Threading) technology.
The rest of the Pentium D's model numbers are fairly simple, the lower the number, the lower the clock. The slowest Pentium D, the 820, doesn't feature enhanced SpeedStep, perhaps as it produces less heat than its brothers.
Moving onto the Pentium 4, we have two ranges within this product group. There's the 6xx processors and the 5xx processors. The 6xx processors feature 2MiB of L2 cache along with Enhanced SpeedStep and HT. The 5xx range have 1MiB of L2 cache and other features vary. The extra cache of a 6xx chip will give it a performance advantage over an identically clocked 5xx chip.
The 5xx series is where the model numbers start to get a bit confusing, as there are chips running at the same clock speed, but possessing different features. So, what Intel has done is tried to use model numbers that group the processors by clock speed, then highlight the minor differences. The second digit in the product number is used to signify clock speed.
So, a 52x processor runs at 2.8GHz, 53x at 3.0GHz and so on in 0.2GHz steps up to the 3.8GHz 57x CPUs. A Pentium 4 processor which has a model number ending in '0' features HT technology. If there's a 'J' on the end, there's 'NX bit' anti-virus protection too. Finally, if the product number ends in '1' the CPU has both of these features, plus EM64T (64-bit) capabilities.
Looking now at the budget CPUs from Intel, we have the Celeron D. It follows a numbering scheme similar to that of the Pentium 4, but with a few subtle differences. No Celeron D processor has HT technology. Any model with a 'J' suffix however, does have NX bit technology. The FSB of Celeron D processors is currently 533MHz and there's 256KiB of L2 cache.
With the Pentium 4 we had product numbers ending in '0' or '1'. The Celeron D's have these, and also '5' or '6'. With the Pentium 4 we could distinguish clock speeds by the second digit, we must also look at the third for the Celeron D. For example, '40' or '41' signifies 2.93GHz clock speed. '45' or '46' indicate 3.06GHz. It's easier to see the pattern from the table.
Celeron D processors are available in two packages. There's the classic 478 pin package, or there's the new LGA775 package. If the model number ends in a '0' or a '5' and doesn't have a J suffix, then that processor is available in either LGA775 or 478 pin packaging. With a J suffix, or a product number ending '1' or '6', it's LGA775 only. Furthermore, '1' and '6' indicate the presence of EM64T.