GeForce 4 Mx cards run the same architecture as the GeForce 2 Mx cards. The only difference is that they've been equiped with faster memory, and have higher clocked core and memory. GeForce 4 Ti is completely different from GeForce 3.
I think you meant the GeForce 3 Ti 200 though.
If your going to pick one of those cards, GeForce 3 Ti 200 w/ 128mb or GeForce 4 Ti 4200 64mb go with the GeForce 4 as it is much faster then the GeForce 3 card. You wont notice the difference between the 128 and 64mbs as games scale and most are built for under 64mbs still. However, I'd go out and get the 128mb version of the GF4 4200. They're really great cards. I wouldn't worry about Nvidia dumping any new cards besides the GFFX 5800 because they just dropped the ultra version in favor of moving onto the Nv35 chipset.