PCStats has published an article on Athlon 64 and AMD's 64-bit technology

With the successful introduction of the Opteron processor in late April, AMD completed one half of its forecast entry into the 64-bit processing world. Based on an evolution of the x86 instruction set used by current 32-bit processors made by Intel and AMD, the Opteron is targeted at the high to mid-range server and workstation market. The second processor released under the AMD64 architecture will be the Athlon 64, formerly known as 'Clawhammer,' which aims to bring 64-bit computing power to the desktop and mobile markets. 64-bit Sounds magical doesn't it? Full of the promise of faster and more powerful computing. After all, 64 is two times 32, so there has got to be a performance increase right? Well, yes and no. When we refer to 32-bit or 64-bit processors, what we are talking about is primarily the size of the registers, tiny, high-speed memory areas built right into the chip itself, closer even than the level 1 cache memory. You can think of them as the hands of the processor, in that they hold values to be operated on or combined.

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