Shortly after Intel introduced the high performance (and high cost) Pentium II processor, they came out with a less expensive and slower performing cousin they called the Celeron. It used the same single edge connector (Slot 1) as the Pentium II, but that was almost all that the two processors had in common, at least in performance.
The first generation of Celeron processors, the 266 and 300 MHz models, came with no level 2 cache on the processor. The processor was based on the Covington core. The lack of the secondary cache was a huge performance hit, and although the processor was designed with the budget-minded computer user in mind, system sales were not what Intel had expected.
Intel responded to those sluggish sales with the Celeron 300a, a second edition of the processor that used the Mendocino core and included a 128KB level 2 cache. Inclusion of the cache made the Celeron a dream processor for anyone on a budget.
Of course, Intel wasn't going to have a budget processor compete with its performance CPU, the Pentium II, so they manufactured the processor to use a 66MHz bus, then locked the speed multiplier for a given speed. Thus, a Celeron 300a, while having the same multiplier as a Pentium II 450, wouldn't outperform its bigger brother.
Or would it?
As some enterprising hobbyists discovered, with very little work and enough cooling, a Celeron processor would easily run at the same 100MHz bus frequency as a Pentium II. And, not so surprisingly, the Celeron performed as well was a Pentium II when it ran at 100MHz because the Celeron's level 2 cache is clocked at the processor's speed, instead of at half the processor's speed on the Pentium II. The Celeron also comes with the very same 32KB level 1 cache as the Pentium II.
Since the Celeron processors were designed for use in low budget systems, the early Celeron chipsets (Intel's EX and LX) weren't designed to use AGP graphics. Fortunately, with the introduction of the ZX chipset, that limitation has been removed.
Another limitation that has fallen by the wayside is the ability to run multiple Celeron processors. Pentium II servers and workstation have been available in dual processor configurations almost since the processor was first introduced. Crafty hobbyists discovered that Celerons could also be used in multiple processor configurations by physically altering the processor.
But the biggest breakthrough in high performance, low budget computing came with the retirement of the Slot 1 Celeron and the introduction of the Socket 370 Celeron. The new Celerons look very much like the older Pentium and Pentium Pro processors. They are compatible with the new Socket 370 motherboard, or with the Slot 1 boards when used with an adapter card. The same restrictions with clock multipliers and multiple CPUs exist, but since the processor has been removed from the old single edge connector card, the same workarounds that applied to the prior generation of Celerons now work with the Socket 370 models, but this time they can be implemented on the motherboard. Abit is the first, with their very popular BP6 dual Celeron board, and others will follow. Alternatively, with a dual Slot 1 motherboard, a set of adapter cards can be used, and most of those include jumpers to enable multiple processing.
Overclocking the Socket 370 Celerons is somewhat problematic. The 300 to 366MHz processors work very well at 100MHz, resulting in processor speeds of 450 to 550MHz. But the Mendocino core Celerons tend to become unreliable at speeds greater than 600MHz...the core just wasn't designed to run that fast. Processor speeds between 66 and 100MHz can cause problems because the PCI and AGP bus timings will be off somewhat. The introduction of the Coppermine-based processor may solve that problem, since that core is designed for speeds of up to 1000MHz.
Of course, since the lower speed Celeron processors overclock so well, they are always in demand. The 300MHz processor is no longer in production, 333's are headed out of production and 366's are always in short supply. Many performance enthusiasts are turning tot he Celeron 400MHz processor as a candidate for overclocking. It seems to work, but because of the high processor speed (600MHz), it gets very hot.
A word of warning is always helpful when it comes to overclocking a processor. All processors generate heat as a byproduct of their operation. The faster the processor, the more heat they generate. Other components in your system generate heat as well. Your video card may have a good sized heat sink on it and, if you have a BX chipset-based motherboard, look at the heatsink on the main chip. They get hot! And once you turn up the clock speed, things are going to get hotter. Be sure that you purchase a good heat sink with a fan on it. You got a great deal on your Celeron processor, but don't scrimp and buy a cheap $5.00 heat sink. Spend a little, get a heat sink with plenty of surface area, a fan that blows towards the processor, and be sure to use some sort of thermal compound to firmly connect the processor with the heat sink. Your system will run cooler and that translates into longer life for your processor.
In conclusion, the Celeron processors, especially the Socket 370 models, offer a great, low-budget opportunity to design a really thrilling high performance system. By working around the design features of the CPU, you'll be able to build a computer that rivals, if not outperforms, a Pentium II or Pentium III system costing much more!