I can give you a brief summary and then point you in the the direction towards more information. The differences between the Athlon XP and the Athlon 64 are relatively small, except for 2 major components.
1. The Athlon 64 has an onboard memory controller. Traditionally, the memory controller is a part of the northbridge, but this is not the case with the Athlon 64. This means that memory latencies are extremely low and the processor can utilize more of its theoretical bandwidth. The only downside to this is that if you want to support new memory types, DDR-II for example, you'll have to replace your processor as the memory controller has to be physically changed to support the new memory.
2. The Athlon 64 has HyperTransport links. These links are essentially high-speed buses to the rest of the system, whether it be AGP/PCI, other I/O, or other processors. The beauty of HT is that it is extremely scalable, which is perfect for a multiprocessor environment.
The Athlon 64 also has a longer pipeline, bigger TLBs, and a bigger scheduler compared to the Athlon XP. It also has additional registers and is built on AMD's .13 micron SOI manufacturing process. The other difference between the Athlon XP and the Athlon 64 is, of course, AMD64, formerly known as X86-64. This allows the Athlon 64 to run 32-bit programs while also having support for 64-bit programs in the future. And those, in a nutshell, are the differences between the Athlon XP and the Athlon 64. Hope this helps
Here's a link to some more info as well:http://www.anandtech.com/cpu/showdoc.html?i=1815&p=2
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