Microsoft is making a version of this operating system for those running x86-64 processors. Currently, that means anyone running the AMD Athlon64/FX/Opteron (hence the "extended" wording in the name). The short version of the name implies the likes of the 386 moving x86 to 32bit from 16bit, and the first 8086 used in the original IBM PCs extending the original architecture from 8bit to 16bit. AMD increased the size of the registers, added a few instructions, and kept its binary compatibility with all previous x86 programs.

The real XP 64 Bit Edition is meant for Intel's Itanium, running the IA-64 instruction set. That's their current 64-bit processor, although with the announcement of the Xeon CT (Yamhill), it seems that Intel is joining in on the x86 extension to 64 bit. But that's a topic for another article. I'll be going in to this more in a separate article on various Instruction Set Architectures.