I finally gave in and built a Pentium Pro system after spotting a CPU/RAM/Board/Heatsink bundle. I started working in IT with NT4 Pentium Workstations and Pentium Pro Servers. I missed out on ever owning one as once I had saved for a computer the Pentium II had arrived, making the Pro rather redundant.

Hardware:
Intel PR440FX Motherboard (Revision 657173-504)
2 x Pentium Pro 200MHz 512K
4 x 256MB 50ns EDO DIMMs
HP Nvidia NVS 100 Quardo4 64MB PCI VGA/DVI
HighPoint RocketRAID 1640 SATA controller
Onboard Intel EtherExpress Pro100/B LAN
Onboard Crystal CS4236B-KQ Audio
Onboard Adaptec 7880P SCSI
IBM Branded Plextor PX-40TSI (68-Pin SCSI)
GoTek Floppy Emulator
Corsair RM850 PSU
512GB Samsung 870 EVO SSD
Creative/Panasonic Audio Cable for CD Audio (Not in pictures as it took a while to track down)
2 x Noiseblocker ITR-PR-1 BlackSilent Pro Fans. Luckily the screws for these are just right to get them onto the existing Pentium Pro heatsinks I had. They are very low power so I could use the existing fan headers on the motherboard.

Case:
Phanteks Enthoo Pro (Tempered Glass Version)
2 x 140mm LTT Noctua PWM Fans. I've had these for many years, but they were too loud in use.
Noctua NA-FC1 4-Pin PWM Fan Controller. To make the fans quiet!
3d printed I/O blank. To make it look a little tidier. I see newsgroup posts from people looking for the I/O shield when the board came out, I assume the chance of finding one now is almost zero!

This was quite a challenge as the PR440FX is almost E-ATX, but not quite. A number of modern cases do not have the clearance at the right side of the board while still being E-ATX (Or SSI EEB these days)

Software:
As with my previous build I went with the quad boot
FreeDOS
Windows 98SE
Windows NT4
Windows 2000

Issues:
This build was more painful for software than my previous build. There were frustrating moments trying to get various OS happy at the same time, but apart from the USB issue mentioned below I did get there.

No ability to skip RAM test! Although it does go slightly faster with the 50ns RAM.
The BIOS is terrible, both when using PnP and somehow worse with PnP off. You can't disable the onboard devices and the Crystal audio is extremely fussy for IRQs etc.
NT4 would not detect the HighPoint controller unless I set MPS to 1.1 instead of 1.4. Once NT4 was up to Service Pack 6, I was able to set MPS back to 1.4.
Windows 2000 doesn't work with the USB. I could get it working if I set IRQ mapping to "ISA Legacy IRQ" but this disables APIC which I don't want to do. Everything else works in DOS/98/NT4 so I'm fine without USB in 2000.
You can boot into Slax Linux (I use it to run TRIM) if you boot with the custom command to turn APIC off to avoid a kernel panic.
The only updated audio driver I've got working is NT4 (and DOS of course). 98/2000 work perfectly with the built in drivers, but trying to use any other versions end in pain and no audio.
EMS only works in FreeDOS if I make sure not to have a bootable CD in the drive to avoid loading an extra SCSI bios ROM, if it loads there isn't enough free memory for the 64K EMS section.

The Quadro is rather too late for the machine, but the DVI output is great with my DataPath E2 to avoid the missing lines in VGA mode.

One nice part is the Logo section in the BIOS, so I put a picture in with the CPU specs and it sadly doesn't tell you it's dual processor during boot otherwise.

Pictures: