The Athlon XP is a 32-Bit CPU; the AMD 64's are (I'll bet you guessed it) 64-Bit processors which can also run 32-Bit programs (99% of the software currently available).
Once 64-Bit CPU's become widely adopted (which is well on the way to happening), there will be gobs of 64-Bit programs which will not run on 32-Bit CPU's, but will absolutely fly on a 64-Bit processor.
there are other differences as well. The Athlon 64 is almost a whole different architecture. It has an on-die memory controller, whereas the AthlonXP uses a tradition front side bus to connect to the motherboard's memory controller, for example. There are other big differences, but all of them point to one basic concept: The Athlon XP is last year's model
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Once 64-Bit CPU's become widely adopted (which is well on the way to happening), there will be gobs of 64-Bit programs which will not run on 32-Bit CPU's, but will absolutely fly on a 64-Bit processor.