HI Wakenaam,
MMX is a processor built in technology that allows it to run an additional set of instructions in paralel. Some Socket 7 (Pentium, K6, K6-2/3, Cyrix) old Motherboards may support a MMX processor if they can use 2.8 Vcore voltage...
I am not sure your old system will handle a MMX videocard...

Maybe some more skilled member can add something here ?
Chris