The DDR and SDR versions of the GeForce2 MX run at the same speed - I think
the DDR has half the memory width of the SDR or something (don't understand
the technicalities but I've seen the benchmarks on Anandtech!).
I'm in the market for an MX myself, and like you don't have much idea what
the differences between them all are. I'm currently planning on getting a
Hercules Prophet II as I've heard it has higher quality RAM than some of the
others, and hence will overclock much more easily.
Cheers
Jim