I bought a couple of used parts lately:

P166 (Intel)
Asus T2P4 V 3.0A

The good thing, the P166 does run with 200 MHz without any problems.
The bad thing, the T2P4 does not offer 75 MHz CPU Clockspeed. I think only the T2P4 V3.1 does.
Another bad thing is that the RC5DES Program seem to use advantages of the MMX structure, so without MMX, you are about 30% slower.

P233 MMX (Intel)
PC-Chips Slot7 mainboard, AGP100 Chipset

This is not so easy to explain like with the non-MMX CPU.
The PC-Chips mainboard offer different settings for CPU-Clockspeed, PCI-Busspeed and different voltage settings for the CPU.

The good thing is, 75*3.5 is no Problem and does about 262 MHz.

Setting CPU Speed Voltage MSDOS Win95
75*3.5 262,5 MHz 2.8 V o.k. o.k.
83.33*3 250 MHz ? o.k. o.k.
83.33*3.5 292 MHz ? ? ?
100*2.5 250 MHz ? o.k. o.k.
100*3 300 MHz 3.2 V o.k. nogo

Dual P166 MMX (Intel)
Gigabyte GA 586DX

That was also easy. I bought the mainboard together with the CPUs used. Unplugged the heatsinks, because they were not sitting fit enough. I put on two new heatsinks with fan and used thermal compound between the heatsink and the CPU.

The printed document has just the jumpersettings for up to 3*66.66 MHz, so I installed it.
The System did boot, I had some real low quality old PS/2 Modules with 4*8 MB, I had trouble with them because they did not run with 60 ns settings in the bios and I always had to reset the bios settings with a jumper, because the machine did not set the settings back, it was just a black screen available.

I plugged in my ST5850A IDE Drive with SUSE Linux 6.0 (Kernel 2.0.36) and get straight into /usr/src/linux, uncommented the SMP option in the Makefile and did a make menuconfig, where I switched off useless stuff and let the machine compile a new Kernel. This run very fine and after maybe 10 minutes, the new Kernel was ready. I booted with it and I do now have 1 Mkey/s with RC5 and at boottime the Kernel says like "930?" Bogomips and the utility "top" do show 99.5% user, 100.7% system, 199.2% nice, 0.0% idle
load average 2.00, 2.00, 2.00

After that I checked out a search engine for "jumper settings 233 MHz GA 586DX" and found a page on the Gigabyte homepage in japan. It was the page for another Gigabyte mainboard but I jumped from there to a page which stated the jumper Settings for my mainboard with use of the newest available BIOS. So I changed the dip switches and the System is now running at 2*233 MHz.

The bad thing is, this machine is now a little bit noisy and I guess it consumes a lot of energy and do produce a little bit heat and dust, which makes it counterproductive in small rooms (like mine).

CeleronA 400 Socket370 (Intel)
Diamond C300 (Micronics) Slot1 LX Chipset

I bought the Diamond C300 used. It run out of the box but detected the CeleronA 400 as a CeleronA 333.

I then flashed the Bios, now the detection was correct but there was no difference in speed, so at the first attempt the CeleronA 400 did already run at 400 MHz but the bios was not able to show more than 333 MHz for the Celeron CPU at boot time.

This mainboard do not offers jumpers or dip switches. The Settings are within the BIOS. Aint no matter what I do in the BIOS, the CPU is always running at 400 MHz. The display at boot time is the only thing that changes.

CeleronA 333 Socket370 (Intel)
PC-Chips mATX MLR 741 (?)

This is the right mainboard for me, it is small, fits in mATX or ATX case and has everything onboard, except ISDN/3D-Acceleration. Everything means: Network, Sound, Graphics adapter, Modem, connectors for floppy, 2*EIDE, 2*serial, 1*paralell. 1 shared ISA/PCI slot. Socket370 and Slot1.

The possible values for the CPU-Busspeed are 66.66 or 100.00 MHz. The Problem is, if I change the multiplyer, the CPU is still running at the original speed. If I change the Busspeed, the CPU is not taking it. 66.66*5 is 333. 100*5 is 500 and this is to much for this CPU.

The setting of 100*4 do also lead to 500 MHz. So it looks like I have no chance to overlock this CPU with that mainboard.

This is the draft for this page, I come up with more accurate information as soon as possible.
Check out my Searchengine: http://www.tuco.de/