General Information

• The first Intel Pentium III processors were released in 1999. They were the successors to the Pentium II processors. A variant on the Pentium III platform was also released called the "Pentium III Xeon." The Pentium 4 processor emerged in 2000. It is the latest in the numbered Pentium series, but Intel has produced some different processors (Pentium Core, for example) since then.
  
  

Bus Speed

• Perhaps the most significant difference between the two processor families' internal architecture is the bus speed. Bus speed is like the speed limit on a highway that the data uses to move around. In a Pentium III processor, the bus speed is generally 133 MHz (although there were a few with 100 MHz). The lowest bus speed on a Pentium 4 is 400 MHz, and there are versions with much higher speeds (topping at 1066 MHz for the "extreme edition").
  
  

Processor Speeds

• The speed of the processor is measured in MHz or GHz. The number translates to how many steps the processor can do in one second. So, a 10 MHz processor can do 10 million steps in one second. One GHz is equal to roughly one thousand MHz. There is some variation, but generally Pentium III processors fall into a range of about 400 MHz to 1 GHz. Pentium 4 processors start at around 1.3 GHz and range up to almost 4 GHz.
  
  

Size

• One of the differences in terms of physical appearance of the two processor families is the size. Generally, as technology advances, parts get smaller. This is the case with the Pentium III and Pentium 4 processors. The Pentium 4s are smaller than the Pentium IIIs. It should be noted, though, that early versions of each of these processor types are not configured the same way as the later ones. That means they have different pins and fit into different receptacles on the motherboard.
  
  

Cache

• Another important difference comes in the form of cache. When referring to processors, cache is a type of memory that resides inside the processor structure itself. This memory can be accessed more quickly than system memory can. The more cache a processor has, the better. Pentium III processors had (for the most part) about 512 KB of cache. Pentium 4 processors, on the other hand, start at 512 KB for the low end and work up to well beyond the 1 GB level.