A comment from Christopher reminded me to mention this: today, the 64-bit version of Windows are not sold retail, and that will continue to be the case for the 64-bit versions that ship as part of the Windows Server 2003 SP1 release.  It will be sold to hardware manufacturers, and available via MSDN Professional and Universal. 

Christopher's comment also led me to this must-read article concerning differences between the Intel and AMD x64's beyond those that I had already mentioned: http://www.TheInquirer.NET/?article=16879.  Thanks, Christopher.