440 EX is low cost version of 440 LX chipset - so 66 MHz as you said.

Celeron (Mendocino) 333 MHz has 128 kB full-speed on chip L2 cache.
Pentium II (Klamath) 333 MHz has 512 kB half-speed off chip L2 cache.
They should have equal performance.

But, please tell me what S-SPEC your Celeron 333 has.
Is it a Socket 370 on a Slot-1 adapter?

If you do not have an adapter you may go up to Celeron 433 MHz (?)
If you have an adapter you may go up to Celeron 533 MHz.
https://www.cpu-world.com/CPUs/Celeron/ ... cino).html
However, that depends on your BIOS does not go mad with that CPU.

I would keep it. It is a good match.

thanks, yeah thats what I thought but the knowledge that more speed (at stock setup) can be gained is giving me the itch to upgrade.

The Celeron 333 Mhz is not in an adapter. I can check the s-pec and report back later.

I also have a Pentium 2 400Mhz in slot 1 but putting it in and runnig it at 66Mhz FSB will result in only 266Mhz speed so its a waste and I assume a Celeron 333 will be faster anyways. Unless I pick 75Mhz FSB to run it at 300Mhz ? RAM I am using is 100Mhz.

I guess Ill leave it as is unles I can come ácross a cheap P2 333Mhz

Another question I have is around slot 1 retention clips / brackets.

I dont have much experience with Slot 1 from the 90s as all machines I was using back then had standard socketed cpus.

I noticed that the current Celeron has a cooler / heatsink on it that "clips" into the two retention brackets on the sides.

This means that cpus like this one:

https://i.postimg.cc/4GDMF8HD/p2-300.png

can not ne inserted without removing the side retentio clips.

How can such CPU be installed in board like this one? would I need to get a bracket / retention clips like this ?

https://i.postimg.cc/vGxfjvnY/bracket.png

I would give a Slot 1 Mendocino Celeron 433 a try - if you can get it easy.
Your Deschutes Pentium II probably won't start or run stable in your motherboard. It is likely your BIOS won't have the microcode updates for that Deschutes Pentium II.

Basically the CPU may also fit without retention clips.
Does your board have a retention set installed?

Yes the board has a retention set but its different to the one I pictured above and the existing celeron 333 has a heatsink with notches on each side where the retention clips "clip on" when pushed into the socket./

Looking at ebay, I can see this - Intel Celeron 400MHz, Slot 1, SL39Z relatively cheap, cant find anything locally (as in Australia) for 433Mhz variant. Its not cheap but might be worth it as its 20% jump in computing power ?

I can get a s370 Celeron 433MHz, SL3BS 66Mhz FSB and also a slotket but it wont be cheap (more than double price of the above mentioned 400) and I am not sure if its worth the effort over current 333Mhz Celeron, outside of having fun configuring it all and making it work of course (which is big part of the reason why I am tinkering in first place).

I doubt it's worth it.