The difference between Athlon 64 and Athlon XP is pretty straightforward, but there are a lot of little differences as well.
Athlon 64, as the name suggests, can process both 64 and 32 bit applications. The Athlon XP can only process 32 bit applications.
The Athlon 64s have their memory controllers built directly into the core, so the FSB technically does not exist on them. The equivelent of the Front Side Bus in the Athlon 64s is called HyperTransport, which can have speeds up to 1Ghz (this is good).
Certain Athlin 64s, based on the Winchester core, also use a smaller transitor core, which is 90 nanometer vs. the more common 130 nanometer. The result is less voltage used by the core, therefore the core stays cooler (this is also good).
If you are looking for the best price/performance ratio, and are willing to sacrifice a faster HyperTransport, dual channel memory capaiblities, and upgradability, you will want the socket 754 3400+.
If you want a faster processor with all the bells and whistles, the 3500+ on socket 939 is the one you are looking for.
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