Well, the performance difference may not be much, but the hardware support difference is.
IIRC, everyone bashes the GF4 MX series because it doesn't have pixel shader support (dunno about vertex shaders...), and therefore acts like a GF2 with shiny new packaging and a misleading name, lol.
So, in the event that I absolutely had to buy and use an nVidia card (God forbid, LOL!), I'd definitely NOT go for the MX series.