Celeron 1.2Ghz ok?

Your motherboard only supports a 100MHz FSB. There are two versions of the PIII 1GHz desktop chip: the common 1000EB and the rare 1000E. The more common chip has a 133MHz FSB and performs much better than the more expensive 1000E, which has a 100MHz FSB (it's only expensive because it's a special chip for upgrading older systems like yours).
The only Celeron to perform adequately is the 1200 you mentioned. It differs from all other Celerons in that it's based on the Tualatin core and has 256kb L2 cache. It's a pretty good performer and will probably outdo the PIII 1000EB. It will definately beet the PIII 1000E.
There is only one way to make it work on your motherboard, you must purchase the Powerleap iP3t adapter to use it. A complete kit with the Celeron 1200, iP3t adapter, and cooling fan is available directly from them for $169.

What's the frequency, Kenneth?

Celeron 1.1 GHz is still the old Copermine Celeron with 128k cache, beaten up regularly by the PIII 866. The 1.2GHz model is a Tualatin, but is still significantly slower than the 256k PIII Tualatin because of it's slower bus speed. IN fact, it's still smaked around a bit by the 1.2GHz Duron! You have to overclock the Celeron to get the necessary bus speed to make it perform at a higher level, but I don't think you can raise the core voltage with the iP3t unless you do pin mods to the processor itself.

What's the frequency, Kenneth?

A reasonable alternative if he wanted to go throught the extra work.

What's the frequency, Kenneth?

With the processor upgrade, he doensn't have to choose a motherboard (and live with that decision), decide between a good cheap motherboard (such as the K7S5A) which may be defective (more time waiting for a replacement) or a more expensive unit (which in the case of VIA chipsets may mean an incompatability with one or more of his older cards), assemble the system, save everything off his drive, reformat and reload all his programs, restore all his files, etc. Plus he would need a new power supply.
Replacing the processor is indeed much easier. I would personally go with a new platform, but then again, that would be a personal decision.

What's the frequency, Kenneth?

RE: Crashman> “<font color=green>...which may be defective (more time waiting for a replacement) or a more expensive unit (which in the case of VIA chipsets may mean an incompatability with one or more of his older cards...</font color=green>"

Heh.. I understand that pretty good… I so love one of my oldie, the Abit BH6… The PowerLeap replied me with the confirmation that they tested OK their PL-iP3/T™ on my particular version v1.01.
I heard from some technicians and read the articles, pointing that a BH6, or even 440BX in general don’t work well with memory in non-native 133 MHz mode.

Well, mine BH6 is c/w Pentium II – 266…, now o/c up to 333 MHz @133 FSB, latest BIOS upgrade, CPU speed signal recognition set to low, CPU pin-21 is disabled.
384MB of RAM set to CAS2. Very stable, never crashed in Win2K.
NIC is working, dial-up modem is working, sound no problem, 20+40 GB HDD, separate boot into Win2K Advanced Server and Win98SE, that’s my Oracle8i training machine.
I don't mind of buying a complete new system, but I also have a pleasure from keeping a reliable veteran running.

As I know, all Intel's motherboards are good and reliable.

That's a good solution also. For example, there is an Athlon-1200@200 + FIC-Az31 + 256MB of PC-133 upgrade combo for only CND$269 (at least 1.5 times less in US$) in our local store. In case of incompatibility with your software - no problem, cause there's a 7-day return policy.