Right now even uder 1600x1200x32 the difference between 32MB and 64MB is extremely minimal and at any lower resolutions/color depths the difference is virtually non-existant. Even overclocked the GF2MX is still far below the performance levels offered by a GF2 GTS.
Due to memory bandwidth limitations the 200MHz core clock on the MX 400 is agin offering virtually no performance boost over the regular MX cards.
I have to agree with the others, a 32MB GF2 GTS is a far better buy then a 64MB MX card, even assuming your able to overclock it heavily beyond spec.