I'm pretty sure the speed of the RAM on most 128MB 4200's is rated at 4ns. This translates to about 500Mhz. Since the 4200's are made to run at 444Mhz, you've got some overclockability headroom on them. From the reviews I've seen, pushing them over the 500Mhz mark seems pretty easy.
Secondly, the 4200 64MB card and the 4400 card both come with 3.6ns RAM I believe. This is rated to run at 550ns.
Deciding on whether to get the faster RAM of the 4200 is a tough decision right now. Personally since you can get a 4400 for only $30 more than a 4200, I'd say get the 4400 and the faster RAM. A harder decision would be between the 64MB 4200 and the 128MB 4400, with a price difference of about $70 now. Since you're considering a 128MB card either way, go with the 4400 and the faster RAM.
Lastly, in the reviews done by Anandtech, it's pretty obvious that the 4200 spanks even the high end GeForce 3 Ti500. There's no reason to even look at that card now with the release of the 4200.
Anandtech has some pretty good info on the differences in the cards and their benchmarks. Now if only they would review a bunch of the 128MB 4200's...