ENS?... LOAD BALANCING?... 64K SIM CARD?...
ENS = Enhanced Network Selection: A capability used by Cingular to evenly distribute customers between the Cingular and the AT&T networks/towers. So basically you'll get the best possible signal between the 2 options.
Load Balancing: This one I'm somewhat guessing but I think it's the same as ENS. Just balancing the calls on the proper network so that as many people as possible get a great connection.
64K SIM card allows the phone to use ENS. Supposed to have a better algorithm to search for the best tower. It also allows you more storage for your phonebook/text messages/etc.