FM Towns:CPU Upgrade for Desktop Models

Basic information
Desktops have a slot to place CPU upgrade cards with some later models requiring the CPU upgrade card to be present as there wasn't a CPU onboard the motherboard. There was absolutely zero retail software that required anything more powerful than a 486DX2 CPU and the majority of the software was designed for the base 386 models in mind. The white desktops with 4 expansion slots are optimal as some CPU upgrade cards were taller and designed for them. The white desktops with 3 expansion slots are shorter and thus more limited on CPU upgrade cards. The 486DX4 cards for the Towns MX/MA type desktops will work on a smaller desktop (EA/ME). However, you won't be able to close the top lid. A good rule of thumb is that CPU upgrade cards made by third parties such as Urban Corporation will fit in any desktop while Fujitsu cards were made for specific models.

Spotting cards in auctions
Sometimes sellers on Yahoo Auctions Japan will have the lid taken off of the computer. This is a good opportunity to see if a CPU card is present or not. It will also generally raise the price.

List of CPU Upgrade cards divided by Desktop type
In addition, the Pentium models can accept the following CPU upgrades.
 * FMT-361 Official Fujitsu Product. For HR desktops only. 486DX2 @ 40MHZ
 * PK-FM486 By I-O Data Equipment. For HR desktops only. 486DX2 @ 40MHZ
 * (Unknown Model number) By SoftBank Corp. For HR desktops only. 486DX2 @ 40MHZ
 * FMT-3621/3621A Official Fujitsu Product. For ME desktops only. 486DX2 @ 50MHz
 * FMT-3623 Official Fujitsu Product. For MA/MX only. 486DX4 @ 100MHz
 * FMT-3622/3622A Official Fujitsu Product. For EA/MA/MF/Fresh only. 486DX2 @ 66MHz
 * ODBT486F By Urban Corporation. 486DX2 compatible CPU (5V) support. Should fit most desktops except HG/HR
 * ODBT486Z By Urban Corporation. The Holy grail and best card as it supports 486DX4 CPU (3.5V) and can use a 5x86 CPU. Often sells for $100-300+ on auction. Fits all desktops except HG/HR.
 * ODBT486 By Urban Corporation. 486SX/DX/DX2 compatible CPU (5V) support
 * FMT-3651 Official Fujitsu Product. For HA Pentium only. 60mhz.
 * FMT-3652 Official Fujitsu Product. For HB/HC Pentium only. 90mhz.

There are also reports of users putting a K6 CPU inside a Pentium HC model.
 * PentiumODP(120/133MHz) For HB Model only. Overdrive processor.
 * PentiumODP(150MHz) For HC Model. Overdrive processor.
 * PentiumODP(166MHz) For HC Model. Overdrive processor.
 * MMXPentiumODP(166MHz) For HC Model.
 * MMXPentiumODP(180MHz) For HC Model.

Good Information
Hota-san has a very good page explaining some of the CPU upgrade cards.