Straight from Microsoft's website, AMD 64-bit processor owners can now download for free a beta version of the upcoming Windows XP 64-Bit Edition, not a bad move by the software giant:
Welcome! If you have a 64-bit ready PC, you now have the option to receive trial software for Windows XP 64-Bit Edition for 64-Bit Extended Systems via CD or download. Read below about the different options for obtaining the pre-release software, and then use the links on the right to register to order the CD Kit or download product files. Please note that in either case, the pre-release software is time-limited and will expire in 360 days.
Important: Windows XP 64-Bit Edition for 64-Bit Extended Systems is only compatible with 64-bit AMD Opteron- or Athlon 64-based computers. It cannot be successfully installed on 64-bit Intel Itanium-based systems.