The turbo switch allows you to do similar to below. Depending on your motherboard jumpers, this can allow you to change clock speed, although a reboot is usually required.

On
==|
Off
|==

What's your motherboard?

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That board should do wonderfully. Personally, I'd source another DPST switch and use a P55C (Pentium MMX) 233.

Put one switch on JP11 so it can be set to 1-2 or 2-3. Jumper JP12 at 1-2. This gives you multiplier options of 2x and 3.5x.
Put one switch on JP2 so it can be set to 1-2 or 2-3. Jumper the upper two pins of JP3. This gives you FSB options of 50mhz and 66mhz.

As a result, you'd have clocks available at 100mhz, 133mhz, 175mhz, and 233mhz along with cache and register manipulation through BIOS and SETMUL. Except for the very slowest stuff, this would be a superbly flexible DOS machine.

http://www.elhvb.com/mobokive/Archive/Soltek/ … 53a2/53a2a5.pdf

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You could definitely control it with an Arduino, although I'd recommend driving small signal relays to switch the headers and provide some galvanic decoupling.

I tried a P54C for this sort of application and I was actually unimpressed. Some of the T12 registers you can manipulate with SETMUL don't have the same impact as with a P55C, so I actually prefer a PMMX for flexibility.

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Small correction: I meant TR12 registers, not T12.

I'm somewhat in the same boat; the board's pins for the turbo switch are not present but probably would work if I get up the nerve to test it out.