Are you not interested in disabling cache on your 166MMX or 486? They would drop down to 286-386 speeds, thus giving you a free 386 😀 If you're itching to build a real 386, then disregard. 😀

Well the cache disabling issue is kinda complex. The 486 is an IBM PC300 series business machine, so it doesn't have any cache memory and it has a fairly limited bios. I'm not sure if I'll ever get around to installing a cache, and I'm not sure if I need to considering the MMX can simply handle anything the 486 can't.

The MMX is a different story, when I disable the cache on it I tend to get fairly strange performance issues. Some games like Wing Commander still run way too fast still(like 486dx speeds) while other games like Doom turn into a slide show(think 386sx speeds). I'm definitely not sure, but I suspect it has something to do with the system having 64mb of ram (tons and tons for 1996) and games that really take advantage of it like Wing Commander just don't slow down properly. But I'm definitely not sure about that. Nevertheless, I haven't too much luck using the MMX as a circa 1990 machine.

Also I have a sweet horizontal AT case with a turbo button that I'm dying to use, and like I mentioned a bunch of unused ISA cards/stuff. And 386's are cool. So I'll probably build the machine. But I am definitely interested in suggestions on making the MMX more versatile too.

If you haven't yet, try setmul. On the 486, you can use it to disable L1, which should get it into 386 territory. On the Pentium MMX, if you are lucky and have a cpu with test registers, you will have a huge gamut of slowdown options.

If you really want to do this just really be sure to shop around so you don't end up over paying or getting ripped off by some of the assholes out there these days. If you are paying top dollar for a board be sure that the barrel of cancer has been removed and that there is no acid damage before you buy as well the board is tested but if you are like most of us just be ready for diy repairs. Most 386 boards are just bog standard without too many differences aside from some having better memory bandwidth but for the exotic stuff you will probably know when you see it plus there are threads here of people showing off theirs. My personal favorite boards are the hyrbid boards but most are isa only while there are a few rare vlb hybrid 386 boards. The only catch is finding them and the price but don't over pay some crazy amount as there are boards out there that can be had reasonably cheap $50 or less shipped.

There was recently a similar thread on this topic: Best 386 Motherboard?
You might find some useful information in that thread.