There are several ways to look at it.
The Ti4600, though most expensive, will yield the highest absolute performance; it will overclock to the highest speed, and it has the fastest type of memory; a full 128MB of it.
The Ti4400 will overclock to the default rate of a Ti4600, maybe a little more even.
The Ti4200, however, may be your best deal; but only if you get the 64MB version, not the 128M version. The 64MB version has faster memory than the 128MB version; thus, it can overclock higher than the 128MB version. Of course, depending on your application, the 128MB version may be faster for you if you are not planning on overclocking. IF you do plan on overclocking, the 64MB's ability to go higher will offset the advantage of the extra memory, not to mention it is about $30 cheaper. The Ti4200 will surpass the base performance of a Ti4400 when overclocked; if it can reach a Ti4600, that I'm not sure. But hey, it costs a heck of a lot less; either way, I doubt the performance will disappoint.
-Ed