I still have 1 Asus P3C-E, 2 Asus P4T533-C and one Intel D850EMV2.
I like RD-Rams, they work fine, with ECC. They are not slowing much when using ECC mode. (About %1~2)
To use ECC, there is not much option from SD-Ram side. Without registered ECC only for servers, not possible to find ECC only Ram for them. Checked eBay today, none! (But maybe very small densities, 32, 64MB etc.) I have looked above 256 MB.
820, 850 and 850E chipsets can be used starting from Windows 95 and later. 820 chipset can go with Windows 3.1 I did not try it and I have no need to test with Windows 3.1 but, I know that, Intel 815 chipset has it's Windows 3.1 drivers for onboard graphics. So 820 may run Windows 3.1. I currently have no AGP cards for Windows 3.1 but can find PCI cards. I know Matrox G400 has Windows 3.1 drivers. I have checked nVidia, their website can not found any Windows 3.1 drivers. I did not check ATI.
But, know that Windows 3.1 driver having cards - DO NOT HAVE DIGITAL MONITOR OUTPUT - They have only analog output, 15 pin D-Sub! (Exception: Intel's own 815 chipset mainboard with digital output "module" for DVO plug.)
So if your interest is not with Windows 3.1, 850 chipset is really fine. If you have a 850 chipset with at least 1 GB you can even install Windows 7 as well.
Windows 7 dropped support for non SSE2 processors in 2018, so you "can not" finish installing all updates from internet with 820 chipset. (All Pentium 2's, 3's even Pentium Pro's on Slot adapter) You can successfully install Windows 7 fine, it gets some online updates good, but later screen goes black, after restart, because of the SSE2 requirement! With 850 & 850E chipset, you can also use Windows 7. But, not Windows 8 or Windows 10, because the processors of 850 chipset do not have NX-bit.
Shortly, you can use 820 chipset starting from Windows 3.1 to Windows XP. (Windows 7 problematic.) 850 chipset starting from Windows 95 to Windows 7.