Here is a quick list of some easy justifications for moving your datacenter to 64-bit.
1.    You no longer need to pay for the licensing costs of Server Enterprise editions to access more than 4GB of RAM. The Standard edition of Server 64-bit is the exact same price as the 32-bit version, BUT, the 64-bit version can address up to 32GB of RAM versus the 4GB that 32 bit Standard is limited to.

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2.    Decreases the demand for I/O capacity on SQL and other database driven applications. With the increase in physical memory support and addressable memory space, the size of caches increase allowing for full utilization of hardware-- this improves overall I/O performance while decreasing the I/O need. In certain Microsoft documents, 64-bit databases on servers equipped with appropriate amounts of physical memory are mentioned as running nearly entirely in RAM—incredible!

3.    Compatibility with 32-bit applications makes the transition easy. 32-bit applications run within an isolated execution layer known as the Windows on Windows 64 layer, (WoW64). This makes the migration from a 32-bit environment to a 64-bit environment much easier. HOWEVER, you should always thoroughly test your applications in the 64-bit environment before putting them into production—especially the applications that tend to be a little bit fussier.

4.    Terminal Services / Citrix XenApp (Presentation Server) environments can see much higher user density numbers per individual server. Regardless of what Server edition you are running, all 32-bit versions of Microsoft’s Server OS are limited to 2GB of RAM for the kernel— no matter how many users are connected, or how much physical memory exists.  64-bit versions lift this limitation into the Terabytes.

The technical benefits provide the ability to immediately see a larger return on investment, consolidate servers, cut operation costs, and provide better performance to the end users.