Can I upgrade from Windows XP (any edition including x64) to a 64-bit version of Windows Vista?
If you have any edition of Windows XP, including the x64 edition, then you are eligable to upgrade to an an equal or better edition of Windows Vista, including 64-bit.
However, if you are upgrading to a 64-bit version of Windows Vista, you must use the upgrade disk to perform a clean install of Windows, even if you own Windows XP x64 edition - you cannot actually perform an upgrade.
In other words, owning a copy of Windows XP allows you to buy a discounted upgrade version of a 64-bit edition of Windows Vista, but you cannot actually use the 64-bit upgrade disk to perform an upgrade - you must clean install.
More information: Microsoft Upgrade Information (external link)