Personally I wouldn't purchase any of the GF4 MX versions at all. They are really just crippled GF2's with a bit better performance in some things than the GF2.
If you are comparing the GF4 MX to the GF3 TI200, I would go for the GF3 TI200 cause they have the same features as the GF4 (directX8, Pixel shaders, etc).
Go with the GF3 Ti200, you will be glad you did when comparing to any of the MX's, but if it's the GF4 TI, I would go for the TI4200 when it comes out. It's a good price point and it should be very fast compared to anything in the GF3 or lower range.