The ability to access over 2 Gigs of Ram was reason enough for me to upgrade to a 64 bit system. My Tenochtitlan project would not have been possible in its current form if I were limited to a 32 bit computer.

Of course, not everybody has the same goals, but some software (like Vue for example) doesn't do as well on older systems. 

Download my free stuff here: http://www.renderosity.com/homepage.php?page=2&userid=323368