With the introduction of 64-bit Linux and Windows versions, there's never been a better time to move to a 64-bit-ready processor. With AMD standardizing on Socket 939 for both Athlon 64 FX and Athlon 64 processors, you have a clear upgrade path for the foreseeable future, while Socket 754 users also have some room to grow their early systems.
For Further Research
The official AMD website for the Athlon 64 processor family is located at http://www.amd.com/us-en/Processors/ProductInformation/0,,30_118_9484,00.html. Follow links from this page to FX, desktop and mobile versions.
These pages contain links to FAQs, technical documents, and architectural briefs.
If you're ready to run a native 64-bit operating system on your Athlon 64-based system, you need drivers. Visit the AMD64 Device Driver Support for Linux and Windows page at http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_869_875^10454,00.html for downloads and information.
Learn more about AMD64 from the official AMD portal at http://www.amd.com/us-en/Processors/ProductInformation/0,,30_118_9331,00.html
The AMD Athlon 64 Processor Power and Thermal Data Sheet also lists processor model numbers, clock speeds, and form factors (Adobe PDF format). Download it from http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/30430.pdf
Tom's Hardware evaluates the performance of the Athlon 64 FX-55 and the Athlon 64 4000+ in "AMD's Athlon64 4000 and FX-55: Nails in the P4 EE's Coffin?" at http://www6.tomshardware.com/cpu/20041019/index.html
Tom's Hardware also provides a useful comparison of Athlon 64 processor form factors in "AMD's Socket 939 Offers More with Much of the Same" at http://www6.tomshardware.com/cpu/20040601/index.html
Anandtech digs into the details of the top Socket 939 processors in "AMD Athlon 64 4000+ & FX-55: A Thorough Investigation" at http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2249