From the w2knews:
Microsoft will ship the Windows Server 2003 x64 Editions this month. They will be sold with new server hardware. Why would you want this? There are some very good reasons actually. First, the 64-bit architecture has more memory address space which means better performance: bigger memory, page files, cache etc.
Second, soon all the new hardware you can get will be 64-bit by default. Third, soon there will be x64-based versions of SQL Server, Exchange, and Office. And Visual Studio 2005 will support native x64 development of code. Fourth, 64-bit machines and software will have a longer shelf life. Think it through for a moment, these machines may not need replacement for 5-6 years instead of the 3-4 year replacement cycle most of us are on now.
The one major problem? Device drivers. You have to make sure that all the devices you hook up to your systems are compatible.
Paul Thurrott's supersite has a good overview of the different 64-bit Windows editions and a great grid that explains which flavor supports how many processors and RAM. Recommended.