As reference, a "PI-like x86" would look like the "LE-366 SBC mini board" equipped with AMD Geode LX 800.

The CPU is a AMD Geode LX 800 @ 500MHz. It's x86 compliant, able to run WindowsCE, Windows XP, and Linux. It's usually comes with installed 256 MByte of ram.

spec and info here. It cost 180 euro when it was a new entry, now you can fine one for 20-30 euro. I have two of these boards, used as embedded SBC to control an industrial sewing machine.

Otherwise, second hand cheap Dell Pentium4 or Intel CoreDuo/Dual mini computers. 50 Euro each.

p.s.
the RDS_on of a mos transistor can be be 0.010 ohm if you choose the right device with the right polarization (1), but if you are afraid of it, you can use an mechanical device electrically operated (aka Rele') which offers an impedance close to zero (0.001 ohm ).

(1) high saturation, narrow channel, with this setup it operates as a current generator, so it has a very tiny impedance

I had a look at “LE-366 SBC mini board” looks like a good board for embedded, but I really need a board that would support 8GB to run Vivado. The x86 boards I found typically support 4GB or less.

It will be a while before I use the racked computers as substitute for a workstation. For now, I just want to get them working together and get used to the environment. There are several pieces of software I would like to get running: cluster tiny basic, civ, asteroids.

I managed to get file sharing from the PI working but I cannot get remote desktop to work. I am guessing it has to do with the userid / password.
For power, I bought some of these modules which have screw terminals for connections.

WayinTop Mosfet Driver Module Dual High-Power 0-20KHz FET PWM Trigger Switch Driver Module DC 5V-36V 15A 400W

I think they are really meant for driving motors, but they should work okay.

Yes, and note that even having a good brand PSU with sufficient specs may not be enough. I was recently working with a friend on a RPi-based C64 drive emulator, and though the RPi was spec'd for 5 V 2 A power, it was suffering mysterious intermittent failures that we eventually tracked down to the PSU, a 5 V 2.1 A Apple model. (This wasn't an issue of power draw from additional logic causing problems, either: the only additional logic was a 74LS06 and a level converter, and the pull-up power on the bus was supplied by the C64.)

If the RPIs would be running Linux and using just Ethernet for connectivity, I'm not sure I see much advantage to using actual hardware rather than just spinning up a bunch of VMs (or even Docker containers, in some cases) to experiment with the software.

Thought I had all the pieces required to wire up the pi rack computer then I found out when I went to plug the power in that the pi4 uses a type C Usb power connecter. This is after I spent an hour hacking together micro-usb cables for power. Maybe not a bad thing as now I am thinking of changing the power supply to a 5.1V model. The PI3 constantly has the low voltage warning blaring away.

Still waiting for the rack case to arrive.

Did you consider using the 40 pin connector instead?

I believe the +5V on the 40 pin connector does not go through a power protection circuit like it does for the usb connector. I want to leave the 40 pin connector available for other pi options. I ordered a standard 5.1V power supply for the pi. It turns out that just purchasing cables and a separate power supply is almost as expensive. I almost bought an adjustable boost regulator for the current supply to beef up the volts, but then I would still need the cable / connector. I may go that route in the future to get 5.2V out.

I managed to get remote desktop working. It was just the password.

I started at home using an XT with 640k ram and floppies, soon followed by a 20MB hard disk add-on. So, the pi seems quite capable to me.

Got the pi workstation up and running with 2x raspberry pi-4 and 1x raspberry pi-3. Played a game of FreeCiv on it. Using remote desktop from the Windows workstation to access it.

Studying MPI.

Still waiting for a case to arrive.

The case for the pi came yesterday. I spent some time today assembling it. Two of the standoffs had stripped threads and were unusable. I lost a couple of nuts. It’s a good thing all the parts weren’t needed for the current computer.