The differences between the Celeron and Pentium III processors can be confusing. In addition, the Celeron chip got bad reviews when it first was introduced in 1998, and that impression has stuck in the minds of many people even though the problem was fixed by Intel in later releases of the chip. Here are the most important differences between the Pentium III and the Celeron chips coming out today:

  • The Celeron chip is based on a Pentium II core while Pentium III chips are based, as you would expect, on a Pentium III core. The Pentium III core contains an enhanced instruction set and has some pipelining improvements that give it slightly better performance on normal instructions. The enhanced instruction set is only useful to programs specially coded to take advantage of it, and there aren't yet that many that do.
  • The Pentium III uses a separate chip for its processor cache, while the Celeron has on-chip cache. This means that the Celeron has one quarter the amount of cache of most Pentium IIIs, but the Celeron's cache runs twice as fast. The first Celerons had no cache and this hurt their performance and gave them bad reviews early-on.
  • Intel manufactures Pentium III chips that run at higher clock speeds than Celeron chips. For example, the fastest Pentium IIIs might be 200 MHz (40%) faster than the fastest Celeron chips.
  • Some Pentium III chips support faster system bus speeds than Celeron chips do.
When you sort all this out and compare the two chips side by side, it turns out that a Celeron and a Pentium III chip running at the same speed are, for all practical purposes, identical in performance. A 500 MHz Pentium III and a 500 MHz Celeron are nearly identical (see the links for some actual comparisons). The Celeron's smaller cache size is mitigated by the speed of its cache, and the extra instructions in the Pentium III go unused in most applications. If you want the fastest machine you can buy, then you need to go with the Pentium III to get the highest clock speeds and the fastest system bus.

Here is a ton of information if you want to learn more: