<< Plus, chances are good that you'll be able to overclock it to 4600 speeds.
>>
I think that's a bit overboard. The Ti4200 runs at 250 MHz core and 222 MHz DDR (128MB version) or 250MHz DDR (64 MB version). This is effectively 444MHz for the 128MB version and 500MHz for the 64 MB version.
The Ti4600 runs at 300MHz core (sometimes more depending on the manufacturer I believe) and effective 650MHz DDR (325MHz X2).
I do not doubt the Ti4200 will be an excellent overclocker, and maybe hit Ti4400+ speeds (275/550) but hitting Ti4600 speeds is a bit of a stretch. I don't think most Ti4200 cores will go that high, and the memory will not be rated close to 650MHz DDR speeds (although definately higher than 500MHz DDR).
Remember that the Ti4200 is actually a shorter card (in terms of physical length of the card), and doesn't have the extra capacitors that the Ti4400/4600 have.
That said, the Ti4200 will be the price/performance champion without a doubt, and still a great overclocker.