Discovered this channel which has some really interesting videos about repurposing thin client mini PCs as DOS retro gaming machines: https://www.youtube.com/watch?v=eQ8GgJfymsc
A follow-up: I bought myself a thin client on eBay yesterday. Has a PCI slot and I also ordered a $5 SoundBlaster-compatible card from China to go with it. Add in a used 4GB CF card for storage and it came to around $30 total
A cool thing I discovered is that there are a bunch of Chinese sellers selling these dirt-cheap PCI sound cards based on the ESS Solo-1, which should be SoundBlaster-compatible and perfect for a cheap retro DOS rig.
The seller I bought my thin client from has one more left in case someone else wants to pick one up and join me in this retro gaming experiment:
https://www.ebay.com/itm/NLynx-Technologies-Minit-6700-301-3700-02-Thin-Client-SEE-NOTES/372277270161
Thin client arrived and I spent some time yesterday installing DOS 6.22 on a CompactFlash card and getting some games running. Quake runs really nicely - I haven't done any benchmarks but it seems comparable to a high-end Pentium
My searches before buying suggested this was going to be a Via Nano CPU but it's actually a SiS 550 - an obscure Pentium clone that's a derivative of the Rise mP6. Hardware is identical to this machine: https://www.parkytowers.me.uk/thin/relisys/rwt205ce/
Overall this seems like the perfect hardware for a mini retro PC. I've tried a bunch of games and the only ones I've had trouble with are the Commander Keen series - the monitor just shows "Out of Range" and no picture. I suspect the monitor is to blame rather than the machine though.
One minor potential gotcha - the PCI slot probably can't accommodate a full-sized PCI card because the CPU heatsink intrudes too much into the space. Fortunately the sound card I've ordered has a triangular shape that should avoid this problem
Got the DOS thin client bridging to my DOSbox dedicated server successfully. "ipxnet connect ipx.soulsphere.org 10000" if you want to try some Quake
Related: did you know that there are four different incompatible ways of framing IPX over Ethernet?
Anyway ipxbox supports them all now https://github.com/fragglet/ipxbox/blob/master/phys/framing.go
an update on the $5 sound card I got: it doesn't work. card detects but I get nothing but very quiet noise out of the speaker socket. that's the problem with buying cheap on ebay: often it's cheap for a reason
My favourite part of this card is the joystick port, which is not actually connected to anything. I was a bit suspicious about the unsoldered pads next to it on the PCB but it's even better: the entire connector is literally not electrically connected whatsoever
There are other ebay sellers selling cards based on the same chipset but without a MIDI port. I'm guessing this particular seller decided to one-up them by including the MIDI port but betting nobody would notice it's just a dummy port
New sound card: Soundblaster Live! CT4780. I had some problems fitting it in because of the CPU heatsink, and this necessitated removing the CD SPDIF connector. I also added a bit of electrical tape to prevent any unwanted shorts.
Everything works with the new card ... except there's no sound output. This isn't completely a joke like it sounds, because it took some work to get the DOS drivers working right. For a while every game using DOS/4GW was locking up on startup. I got past that with the right EMM386 settings but it took some research. Now everything works fine, except no actual sound. Frustrating.
I don't think the problem is in the output circuitry because I can hear a pop on the headphone out when the drivers get loaded. Rather, these SB Live cards have so many inputs, outputs, bells and whistles, and I suspect that somewhere in there, something's muted or set to the wrong output channel. The DOS drivers only expose a handful of settings knobs.
@fraggle Ooof, now that's really old school. Fun fact, Centaur is still around and they're focusing on low-power x86 servers now.
And yes, I remember SiS because I had a PC-CHIPS motherboard that used that chipset for my Celeron 300a whitebox. 😬
@fraggle Oh, it's sideways! It took me a moment to even spot the thing and work out how that could be.
@LionsPhil PCI slot is on a riser board. it's a teensy little compact machine
@fraggle Now you just need a massive old CRT to go with it. 
@fraggle i thought they were earrings for a moment
@fraggle
Getting the real DOS experience of spending days getting sound working, then.
k͕̽/\̲̻͍ͬ̾̽tn͑̿͐ǰi̘͕̒̿å 
@fraggle how did you get it set up with DOS? I have this card too and tried to get it working in FreeDOS with the drivers from Phil's Computer Lab. I haven't messed with hardware and DOS since the 90s, so I kind of forgot everything about IRQ-fu and what not 
@katnjia three things:
1. it's important to choose a non-conflicting IRQ. I used MSDOS's MSD (diagnostic) program to find an unused one
2. I had to add a line for EMM386 in CONFIG.SYS to get the SBEINIT driver to load
3. The EMM386 line needs extra options (otherwise all DOS/4GW games hang) and looks like this:
DEVICE=C:\WINDOWS\EMM386.EXE I=B000-B7FF RAM D=256 AUTO
@fraggle Q. C. Passed!
These machines are low spec (by modern standards) and used ones can be picked up for $30 or less on eBay. The main thing missing for DOS gaming is sound, but some of them have PCI slots which means it's possible to fit a Soundblaster-compatible card in them.