The MX is the deliberate crippled version of the Geforce2 on the memory side.
The MX 400 gives almost identical performance to the MX, the only different is a higher core clock speed and sometimes bundles with 64MB (this doesn't seems to have any advantage)
The MX 200 is uses slower memory than the MX and the MX400, large performance loss there. Try not to go there.
Nvidia Geforce2 MX is around for sometimes now for budget market. The MX400 and 200 is the successor of the MX(MX has been discontinued). Nvidia basically split the MX into MX400 and MX200. As mentioned the MX400 gives almost identical performance to the MX, while the MX200 is a cripple of a cripple version of the Geforce2.
I agree with the Radeon LE, it should be a better choice in most cases. (newegg.com you know)