Quick View of the Pentium III and Celeron CPU

Pentium III
Aside from higher clock speeds, the only essential difference between this chip and the PII is SSE, or Streaming SIMD Extensions. SIMD stands for Single Instruction, Multiple Data. Streaming concerns the transfer of long streams of data to and from memory, very useful for databases. Also included are a few extensions to MMX to speed up video processing, particularly 3D and lighting calculations, assuming your software can use the instructions.

PIIIs made with the Katmai (.25 micron) manufacturing process have a larger (512K) 2-way set associative L2 cache running at half the processor speed. Coppermine is the .18 micron process (look for the E suffix, for Enhanced, up to 600 MHz – the 650 MHz ones all use it – it means Advanced Transfer Cache and Advanced Buffering Support) with 256K of 8-way set associative L2 cache running at full processor speed, because it is now on the die. A B suffix indicates the ability to run with the 133 MHz FSB (533, 600 MHz). 677 & 733 MHz processors don’t have suffixes because they are the only chips to run at that speed, so there is no confusion. All will use Slot 1 initially, but will migrate to Socket 370 (the flip-chip module), as used by the Celeron. This makes it a similar size chip to the Pentium Pro. This sort of design is possible because cache chips are not needed locally or on a daughter card, as the process is down to .18 microns, leaving more room on-chip. This also means a cooler chip, less heating and more overclocking!

Celeron
A cut-down version of the PII aimed at the low-cost market, initially supplied without an L2 cache, which prompted the unofficial name of DeCeleron. It was subsequently reissued with 128K of L2 cache running at processor speed, resulting in a chip that has gained some respect, especially as it rivals the PII in many areas. It started off using Slot 1, but now uses Socket 370, with the provisio that, from 533 MHz, Coppermine (.18 micron) technology was used and doesn’t necessarily fit your socket, as some of the pinouts were changed. Converters are available, though, including those to allow Socket 370 chips to use Slot 1. Although the chip is as fast, if not faster, than PII's or even PIIIs, its front side bus only runs at 66 MHz. Also, you will not be able to upgrade a socket 370 Celeron to a Pentium III (if for no other reason, they don’t use the same voltages). Also, be aware that the 400 and 433 MHz versions use fixed clock multipliers of 6 and 6.5, which means 600 and 650 if you try to use an FSB of 100 MHz.

This is an article from  Phil Croucher, author of  "The BIOS Companion" Phil has a way of explaining in "plain" English. The information is well presented and is well above A+ standard.