Speaking generally this will depend on the motherboard revision.
Older slot 1 motherboards are limited by the VRM to a Katmai 600.

Although given your starting out with a P3 rather then a P2 and you have found someone else who has upgraded I'd think you have a good chance of success.
If it doesn't work the PC just won't boot, it won't damage anything.

To be sure you need to find the voltage regulator chip on the motherboard (a small IC, usually between ATX connector and MOSFETs) and look up its datasheet. If it supports VRM8.4, you're good to go to just stick a Coppermine in.

If it does not support VRM8.4 (so 8.2 or 8.3 compliant), in theory it's 'no', but in practice a 1.65V-1.75V CPU will survive 1.8V - however the board will not boot if the CPU requests a voltage unsupported by VRM. To force 1.8V, you either need to mod some pins, or go for a So370-Slot1 slocket that does that for you. If you force 1.8V, the CPU will still draw the same current as it would at lower voltage. This may overload the VRM, so check that it supports the required current (you can calculate the current by dividing TDP by nominal voltage). If not, don't do it. If it does, it should be good so long as you cool it well enough.

Here's a picture of the VRM on the motherboard:

...and here is the datasheet (I think): https://www.alldatasheet.com/datasheet-pdf/pd … H/EZ1585CM.html

It appears to be an adjustable voltage regulator (1.3V to 5.7V) but it looks like this VRM was available in different configurations (including fixed voltage) so I'm not sure.

Is there a way I can test it before purchasing a faster CPU? I can measure the output voltage using my multimeter after I buy/insert a new CPU but it would be nice to test it before if possible...

Also, can I use the 133 FSB CPUs with my 100 FSB motherboard? (ie. do they just run at the 100 FSB or does it cause other problems/slowdowns?)

this is a VRM, but not the CPU VRM. CPU one is located near CPU slot. Might be U6 (seems small), more maybe chip between CPU and RAM (too far). Basically you are looking for >20 pin VRM controller, not a 3 legged transistor

EDIT: I think I found it: ADP3152 (5-Bit Programmable Synchronous Switching Regulator Controller for Pentium® II Processor) and lists support for 1.8v to 3.5v. VRM 8.2 compatible.

Bummer 🙁

U6 LM80 is a fan controller
U4 W40S01-04 is ram buffer
U40 is our guy, Analog ADP3152. I disregard it at the beginning due to not enough legs for serious VRM controller, but this one supports just one phase. 1.8V and 3.5V
Edit: didnt see your edit 😀

In order to run Coppermine you will need Slotket with Voltage jumpers, set it to 1.8V (overclocking voltage) and hope for the best. You wont damage CPU when properly cooled, but some P3s didnt like higher voltage and crashed under load. Absolute Maximum voltage for Coppermine is 2.1V according to Intel.

Thanks so much for the help! I might look at building another PC. I don't like the desktop (versus tower) form factor anyways of the 300PL 😀