Just little corrections: MMX first appeared on exchanged P5 (e.g. so-called P1) core. It's pretty easy to distinguish Pentium classic from Pentium-MMX only by looking it's top (let's just say Pentium classic has lame surface and Pentium-MMX doesn't ). All newer Intel's processors since Pentium-MMX are supporting MMX instructions.
All AMD processors since K6 (the one that runned on 166-233Mhz) are supporting MMX.
Cyrix processors (now VIA) are supporting MMX since 6x86MX (was it with codename M2?).
Winchip2 (haven't seen that kind of animal myself) supports MMX, too.
Like Chwolf said, MMX support hasn't anything to do with your motherboard, that's plain CPU's feature.