The differences between the Ti range of cards is quite small, the 4200 has the same core but slower memory (different package), the 4400 has slightly lower rated memory than the 4600. With the 4200 you can reach 4400 speeds at a push and with the 4400 you can reach 4600 speeds quite easily (maybe the main reason to discontinue it?, especially as it's cheaper). It also depends on the manufacturer and how they have designed and rated their boards.
The MX range are all DX7 based chips, I really wouldn't go anywhere near an GF4MX, it won't be appreciably faster than what you have now and also you won't gain anything in quality or ability with the change.
Look for cheap GF3Ti500s or Ti4400s (if your budget stretches that far). Otherwise look to getting a Ti4200. Or save up and wait for prices to drop. The ATi 9700 is the ultimate card at the moment, but it is new and untried. If you can wait until the Nv30 is released then do. There will be some competition in the market then which will drive the prices down, and besides nVidia may bring out a killer card
which will drive prices down further.
Now is a very difficult time to upgrade, worse than most times. You would be better off waiting until after Christmas before you make the change unless, of course, you see a really good deal