It is most likely a Socket 3 machine (Could be Socket 1, but that does not matter for upgrades beside it's going to be a bit more tricky to get the CPU loose without the ZIF handle.)

You have a varity of upgrade options. I wouldn't gamble that the board/machine supports 3v cpu's directly, meaning you need 5V stuff. There are Pentium Overdrive 63&83mhz, Intel Overdrive DX4-100 and Evergreen 586, Kingston TurboChip 586 interproser boards with an AMD X5-P75 133mhz. The last option is probably what the guy on Youtube uses. The fastest is the Pentium Overdrive tied with the 133mhz options (depends on your system which one will be the quickest). The Pentium Overdrive is probably the one that is easiest to acquire.

If you already have a heatsink with a fan (or a big heatsink without fan), then that is enough for any new CPU. The CPUs back then didn't give out much heat. The Pentium Overdrive comes with a fan as standard.

What do you think about this one:
http://www.cpu-world.com/CPUs/5x86/Cyrix-5x86-100GP.html
Is that one my socket compatible?
How fast this one is comparing to 486DX66 and Pentium 120Mhz?

Fastest 486 CPUs (133 Mhz) are usually said to be nearly equal to 75 Mhz Pentium.

If you aren't sure what socket you have, you might want to go with so called Kingston Turbochip TC5x86/133, it's compatible with any 486 socket and voltage.

Why can't you determine your CPU version? Is it covered with a heatsink?

Niezgodka wrote:

What do you think about this one: http://www.cpu-world.com/CPUs/5x86/Cyrix-5x86-100GP.html Is that one my socket compatible? How […]
Show full quote

What do you think about this one:
http://www.cpu-world.com/CPUs/5x86/Cyrix-5x86-100GP.html
Is that one my socket compatible?
How fast this one is comparing to 486DX66 and Pentium 120Mhz?

It will fit into your socket, but it requires 3.45V. If your board only supports 5V you can't use it without an adapter like the Powerleap (even then you might have issues with your BIOS). You need to check if there are any jumpers to change the voltage on the motherboard.

I'm gonna take a look again today and search online. Meanwhile vetz, if you can suggest me any particular model...
I checked on ebay and it looks like between $40-$70! I was hoping for some $20 tops! 🙁

The easiest solution is to look for the Kingston/Powerleap/Evergreen 5x86-133, they will be a direct swap for your CPU and you can change the jumpers on the upgrade CPU to change between write back & write through cache.

The next best option would be to get an intel 486DX4-100 Overdrive processor, the one with the black heatsink attached and it is also a direct replacement, not as fast though.

I would forget about running a cyrix 5x86 as in all probability your mainboard wont support it.

edit: You might get an CPU for $20 on ebay but I don't like your chances, you should be able to pick one up for around the $40-50 mark in a normal auction.

dirkmirk wrote:

edit: You might get an CPU for $20 on ebay but I don't like your chances, you should be able to pick one up for around the $40-50 mark in a normal auction.

I agree to this. The Overdrive CPU/upgrade options are not cheap, sorry, but you have to be very lucky to get any of them for 20 dollars.