The Celeron is inexpensive and a decent performer. Particularly the new models (533A to 600 MHz and more) are an excellent choice for performance freaks on a small budget, as they are around only $100-120 and just perfect for overclocking. All new Celeron CPUs (green FC-PGA package) should reach at least 800 MHz, as they do not differ from the Pentium III at all.
We know that the latest Celerons are Pentium III processors, which did not pass the L2 cache test. In order to reduce the number of defective Pentium III CPUs, Intel disables half of the L2 cache (the part where the error is situated), reduces the cache associativity from 8-way to 4-way, restricts them to 66 MHz at higher multipliers and labels those chips as Celeron.
If you are looking for a way to upgrade your Pentium II system, have an eye at the Mendocino Celeron. Older motherboards should easily be able to host this processor as long as you still get an updated BIOS. Celeron models at 500 and 533 MHz can be obtained for less than $100 today and will significantly speed up an old Pentium II 233-333 system.
All Celerons coming in the green FC-PGA package are equipped with the Coppermine-128 core (533 MHz or faster). I don't recommend buying such a CPU without checking twice if your motherboard is able to provide 1.5 -1.65V. You are in good shape if your board can host one of the new Celerons, but even if the voltage requirements are not met, you can still take advantage of a Celeron 400-533 with Mendocino core.