Server 2008 will be the last 32-bit OS that Microsoft will release for servers and clients. For more information, go to http://www.betanews.com/article. If your hardware will support it, I suggest running the x64 version of Server 2008. For Exchange 2007, x64 is mandatory; for SQL Server 2005, x64 will provide significantly better performance on the same hardware compared to the x86 version. In my experience, the performance gains are especially notable with large SQL Server applications. On the 64-bit platform, you can improve performance by adding memory to the SQL Server when you’ve hit the 4GB limit. For more information about performance improvements, go to http://searchsqlserver.techtarget.com and http://download.microsoft.com/downloadf. Of course, you might have legacy applications that won’t run on the x64 version.
One program that currently isn’t supported but will work with x64 is Group Policy Management Console (GPMC) 2.0. As you know, GPMC 1.0 will run only on the x86 version of Windows Server. If you’re fortunate enough to have a server that supports x64 virtualization, I suggest running both an x64 and x86 Virtual Server version of Server 2008. Doing so will let you test application compatibility without having to set up two physical servers. The x64 version of Server 2008 has better memory support and performance than the x86 version running on the same hardware, assuming in the server has adequate memory installed.
In my experience, the x64 version of Server 2008 will require approximately 100MB more than the x86 version. However, just to be on the safe side, I typically configure a server with at least an additional 300MB of memory if I know that server (physical or virtual) will be running any x64 version of Server 2008.