Microsoft has a link to a 23 page, IDC authored whitepaper called Understanding the Business Benefits Associated with x86 64-bit Windows Server, which is a good read. In my opinion, there is no good reason to deploy any new server or system on a 32-bit version of Windows Server 2003 or above. Windows Server 2008 will be the last 32-bit Server OS from Microsoft. Any new server (Intel or AMD) that you can buy right now (and for quite some time) is 64-bit capable, so there is no incremental cost to running 64-bit.
NewsGator has been 100% 64-bit for our production SQL Server 2005 infrastructure for nearly 18 months, and we are not looking back. We have had great success running 64-bit in our environment.