The GeForce2 MX due to the smaller and cooler .18 micron chip might have higher O/C potential for the CPU, but that won't affect performance very much. Overclocking the memory is where its at.
http://www.anandtech.com/showdoc.html?i=1266&p=3
The GeForce 2 MX uses slower memory than the GeForce 2 DDR.
It appears that the MX can either use 64-bit or 128-bit SDRAM. Hopefully it uses 128-bit otherwise the memory bandwidth will be even less than the GeForce 256 SDR. I couldn't find any info on Guillemot's/Hercule's websites about the memory used in their 3D Prophet II MX. The review over at PlanetHardware
http://www.planethardware.com/reviews/videocards/hercules_prophet2_mx/index2.shtml just shows 32MB SDR SDRAM running at 166Mhz and implies its 128-bit, but doesn't flat out state it.
The GeForce 256 DDR runs at 150Mhz DDR and generally overclocks fairly nicely. I'm running mine at 170Mhz and I haven't really experimented enough to find where the boundary is. I'm not very good about noticing snow or other artifacts, so I don't generally push my video cards. Anandtech got the Asus V6800 up to 158/173.
I'm not really sure if the GeForce2 MX really does o/c better, at least where it counts. One might be able to overclock the memory on the MX past 189Mhz, however one would have to push it past 212Mhz to provide the equivalent performance boost of overclocking the GeForce 256 DDR's memory. I'm not sure it will be able to overclock that much. Anyone have any pointers to overclocking the MX yet?