If I recall correctly, I read in the web-nuts.com forum that the 257mb does nothing to improve the service because the system is programed only for 64mb so the extra mb's do nothing. This was written by a person who has a new unit with the 257mb enhancement and was working with it to see if there was any difference.
This sounds logical to me because the entire msntv2 software program is on servers at MSN and coding and software is geared for the 64/128mg boxes which is about 99 percent of the existing units.
The above comments are just my best guess; I'm not a tech nor do I have any inside info about this matter.
Art