Today I searched a little more to find out about SoC devices which incorporate an x86-compatible processor.
These appear to be the main (current) contenders in this space:
- AMD Geode GX / LX, or older AMD Elan
- DM&P Vortex86 and friends (PMX-1000)
- RDC?
- Intel Atom
- VIA (though I can't seem to find their chips anywhere)
- and other Chinese manufacturers / clones which I would have no idea how to source
...of which there are an abundance of little-known manufacturers of single-board computers based around these chips. Most range from $100-250 and come with VGA out, audio out, RAM, etc. A lot of it is, as you noted, designed for industrial markets: these boards get elaborate with 3x COM ports, a huge GPIO header, analog sampling inputs, etc.
This is just one example by EBOX which would be tough to beat as a hobbyist designer, as it seems to offer pretty much everything you'd need: VGA, sound, plenty of RAM, keyboard/mouse connections. http://www.compactpc.com.tw/product.aspx?act=detail&id=139
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There are other solutions which don't involve using a "real" x86 processor, including doing the whole thing on FPGA (Zet project - http://zet.aluzina.org/forums) which could potentially then be implemented into the Replay board from fpgaarcade.com (http://www.fpgaarcade.com/).
Or if you really don't care about hardware accuracy, just run DosBOX on Raspberry Pi. http://www.youtube.com/watch?v=fvztpECEh_0