The biggest difference between x64 and other 64-bit processors is that x64 processors are compatible at the hardware level with 32-bit, x86 processors. There are currently two basic x64 processor families:

From AMD, this includes the Athlon 64, Athlon 64 FX, Mobile Athlon 64, Turion 64, and Opteron processors. From Intel, this includes the Xeon with EM64T and Pentium 4 with EM64T processors. The two architectures are binary compatible, allowing Windows XP Professional x64 Edition to use a single version to support both.

http://www.microsoft.com/windowsxp/using/64bit/russel_exploringx64.mspx

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From Srinath (Coming back !)