So we know how x64 can benefit Exchange 2007 and Terminal Services, but what about one of the most common parts of a Windows network, Active Directory?
Similar to the limitations with terminal services and Exchange, AD suffers from the 2GB virtual memory limit of 32 bit OSes. The more objects in the domain the bigger the problem. With larger domains that contain a large number of objects, it gets difficult to cache the AD dataase in memory and authentication requests and queries leads to excessive paging and a slow down in performance. Again the x64 versions and the lack of a real memory limit allows the entire database to be loaded into memory which reduces paging, and provides quicker access to the data in the database. All this leads to faster processing of authentication requests, faster response to queries, and faster overall performance. Considering that the x64 version of Windows Server 2003 costs the same as the x86 version, the choice is yours :)