The Windows XP Professional x64 Edition Technology Advancement Program enables customers who have purchased Windows XP Professional (32-bit) to exchange it for Windows XP Professional x64 Edition. You will need to have an x64 processor (the AMD Athlon 64, AMD Opteron, Intel Pentium 4 with EM64T, or Intel Xeon with EM64T) to run the new software.
Windows XP Professional x64 Edition Officially Live
Source: PC Perspective
Well, the wait is finally over. Windows XP Professional x64 Edition is finally, a real live, non-beta, official software release today! Current users of the 32-bit version of Windows XP Pro are eligible for a free upgrade, which is very unexpected from a company like MS, so take advantage, but ONLY after you have found all the necessary 64-bit drivers for your devices.