I'm trying to figure out what direction I should take for my next build/project. Currently, I have four retro computers on my desk:

A late-95/early-96 vintage Zenith-branded Pentium 100 Mhz with an OptiViper chipset and a Diamond-branded S3 Trio64V+ and Aztech sound card for DOS 6.22/Win 3.11
A PentiumMMX 233Mhz system on the 430TX chipset with a Creative-branded Riva TNT and Yamaha Audician sound card for Win95c.
A Pentium III 1Ghz system on a Dell-branded i815 motherboard with a Gainward GeForce4 ti4200, Dual Voodoo2 12MBs, and a Sound Blaster Live! Value for Win98SE
A Phenom II X4 940 with a Radeon 6850 and a Soundblaster X-Fi for WinXP and Win 7 (but its hard to think of that computer as "retro")

Component wise, I'm planning to stick a 3dfx card in the PentiumMMX system, and I might "downgrade" to a Riva128 for the primary video card. Beyond that, it's hard to say what I should do, but I keep coming back to a few things:

A 486. I've never owned a 486. I've had a few chances this year to buy 486 motherboards at good prices, but I passed because my P100 runs basically all of my DOS games. If I went 486, I'd probably go for a lower end build, 33Mhz or lower to create some separation on the low end of my systems.
A rebuild/upgrade of my old Pentium 100 Packard Bell running the initial release version of Win95. Yes, it's basically the same thing as my Zenith. However, cache-less Socket 5 PBs are plentiful on ebay for now, and it might be interesting to experiment a Powerleap K6-2 or K6-III if I could find one.
Upgrade the processor and video card in my Phenom II system. The Phenom II X6 1100T isn't hard to find on ebay and would be a solid upgrade for this system. It wouldn't bring the computer into the modern era or provide too much benefit in WinXP, but it would likely give me some more usability for mid-2010s games. As for the video card, it seems Nvidia had better WinXP driver support more recently than ATI/AMD, so, again, some additional power.

A 386 would be nice too, especially if I could find a Cumulus-branded one like my first IBM-compatible computer, but I haven't seen one of those in 20 or so years.

I'm open to suggestions.