Oh, check this out: I have a Cyrix 6x86MX PR233 @188MHz, an equivalent of Pentium MMX @233MHz, in a fully A-Z working case. I guess the problem is just finding compatible I/O devices.
I remember getting this years ago to play games in native DOS mode. Didn't get it too long ago close to its generation, tho.
But it does not come close to my 486DX in age.