Here's a very useful compatibility matrix of Microsoft product and their support for both Itanium and x64.
What is x64 you ask? It is the “64-bit extensions“ technology pioneered by AMD which is in use in some of their Athlon products and all the Opteron. Intel has recently begun supporting this technology in their EM64T (details) series of processors.
Windows XP 64-bit Edition
Windows Server 2003 Web Edition
Windows Server 2003 Standard Edition
Windows Server 2003 Enterprise Edition
Windows Server 2003 Datacenter Edition
MS SQL Server
X indicates product in market today;
Y indicates product scheduled for release in 4Q04
Z indicates product scheduled for release in 2H05