We have tested our product on MS Windows Server 2003 with MS SQL Server 2005 (both 64-bit editions) and it works with no problem. We use only standard .NET/SQL2005 features. Kentico CMS is compiled for 32bit OS, so if you wish to fully leverage the 64bit OS, you may need to recompile the full source code using Visual Studio 2005/2008 for the 64bit environment.