Windows Server 2008 is the best method for Windows 7 64-bit. You can run Windows 7 off Windows Server 2003, but GPO and Active Directory will not have all of the functions available to control Windows 7 client systems. Windows Server 2008 will allow you to control and manage these clients. Best of all, Windows Server 2008 CAL licensing restrictions are not as stringent as in 2003. You may want to look at using Windows Server 2008 R2 Standard or Enterprise for this situation.