In the immediate sense the 64 bit client will not likely make any difference whatsoever, we used it for awhile on Beta and it's not anything noticeable. EQ's current performance problem specifically meaning poor handling of lots of players in a zone, poor server performance during raid events (especially during big burns), poor client performance during same, login server performance none of that is improved by going to 64 bit.
Going to 64 bits raises the amount of addressable memory from 2^32 to 2^64 bytes. JChan basically said that one of the most common causes of crash to desktops is an out of memory exception, in theory this is likely caused by some problem in their code and could be resolved without actually needing to change the application architecture--because when running properly EQ doesn't need anything close to its current 2^32 limit, in fact you can get the client down to way way less than that. But anyway, it will mean the people who frequenlty get out of memory crashes for various reasons won't get them as much, probably.
It doesn't immediatley change much of anything else. It does open some new doors for them in a few areas going forward, and lets them push things in the client they may not have done previously.