I decided to visit my local charity shop this weekend just on the off chance of finding something. I hardly ever find anything there but I did once find a working Commodore monitor so I go back occasionally just to check. There in amongst the boardgames was a box labelled motherboard. The branding on it identified it as a Gigabyte Athlon board, but inside was the board that the Gigabyte probably replaced - an i440ZX Celeron socket 370 board from EPOX with CPU and cooler. There was also an ESS sound card in the box as well as some new IDE cables and the manual etc for the Gigabyte. Total cost less than 1 dollar.
I spent quite a fun morning getting the thing to work. It was a bit dusty but otherwise perfectly functional. It's quite a tidy little ATX board with some ISA slots, AGP and no integrated graphics or sound. I chucked two PC133 DIMMs in to bring it up to the max of 512MB RAM and used a spare 4GB IDE drive with Win98 on it. It makes quite a good little DOS machine - it might even replace my K6 Socket 7 DOS PC.
The ESS card didn't impress me though as I couldn't get any Yamaha OPL sound to work, but if I do use it as my primary DOS machine I'll just use my SB16/DB50XG combo instead.
I used my old GeForce 440MX AGP card for video, but that's not ideal as the cooler fan has seized - hopefully once the 6600 GS I've ordered arrives for my P4 machine I can retire the 5200 that's in there now to the Celeron. Bit of an overkill since the board only supports AGP2X AFAIK, but it should be safer than using the 440MX. Otherwise I can just move over the Voodoo from my Socket 7 PC.