MX roughly means stripped down version.
Imagine a Honda Civic. They got DX for base model, EX, LX, etc... mean they get added neat features.
Now imagine a GF4 Ti4200 as the base model. The Ti4400(or whatever) is the souped up model. The MX would be like putting bald tires, an engine with 2 less cylinders, no power brakes, no power steering, but you do got power windows and a CD player!
-SW