If this:

Is your MB then it looks a lot like Intels own "Advanced ZP" board as seen here:
Intel-built 486/Pentium/PPro/PII motherboard guide
http://tulrich.com/tectrixvr/intel_pba-638995_manual.pdf

And that suggests p120 as max.
But if it supports 2x60 in the p120 and 1.5x66 in the p100 i see no reason it should have any problems with 2x66 and a p133.
No MMX tho, you need to go overdrive for that.

But IMO there is no reason to go to far with hardware like this.
No amount of upgrades will make it fast enough to compete with a generation newer hardware.
So you might as well enjoy it in its original form =)
Just my €0.02

If it's got 66 and 2x and is saying 120 Max then anticipate that it is core current/wattage limited and the regulator is going to need extra cooling.

Great, thank you very much for the responses guys, for me the cmos battery is in the top-right hand corner of the board, but the placement of the slots, CPU socket etc are in the exact same place as seen in @bofh.fromhell's photo. It looks like I'll stick with the p100, now to look for a voodoo 1 card 😀

Check what's written on the CPU socket. If it's a Socket 5 then you will definitively be limited to the slower pentiums ( Usually 133MHz max)