If you purchased an x64 based PC between March 31st 2003 and July 31st 2005 which came preloaded with the OEM version of Windows XP Professional, a trade in program has been established which will allow you to 'upgrade' to Windows x64 Professional Edition for the cost of shipping. Though I would recommend you try the free trial version before taking part in this trade in because once you are licensed to use x64 Edition, your 32 Bit Edition will no longer be licensed.

Free Trial:

http://www.microsoft.com/windowsxp/64bit/evaluation/trial.mspx

For more on the trade in see
http://www.microsoft.com/windowsxp/64bit/upgrade/default.mspx

5 Reasons to use Windows XP x64
http://www.microsoft.com/windowsxp/64bit/evaluation/top5.mspx