If you are currently running: You can upgrade to:
Windows Server 2003 Standard Edition (R2, Service Pack 1 or Service Pack 2) Full Installation of Windows Server 2008 Standard Edition
Full Installation of Windows Server 2008 Enterprise Edition
Windows Server 2003 Enterprise Edition (R2, Service Pack 1 or Service Pack 2) Full Installation of Windows Server 2008 Enterprise Edition
Windows Server 2003 Datacenter Edition (R2, Service Pack 1 or Service Pack 2)

Full Installation of Windows Server 2008 Datacenter Edition


There are a couple of important things to remember here. 

  1. With the exception of Windows Server 2008 for Itanium, the table above applies to both x86 and x64 versions.  However, cross-platform upgrades (x86 to x64 or vice-versa) are not supported. 
  2. It is also not possible to upgrade from a previous version of Windows to Windows Server 2008 Server Core Edition.