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http://athlonxp.amd.com/faq/ :
What do the 2000+, 1900+, 1800+, 1700+, and 1600+ numbers mean?
These are model numbers. AMD identifies the AMD Athlon XP processor using model numbers, as opposed to megahertz, such as 2000+, 1900+, 1800+, 1700+, and 1600+ versions. Model numbers are designed to communicate the relative application performance among the various AMD Athlon XP processors, as well as communicate the architectural superiority over existing AMD Athlon processors. The AMD Athlon XP processor 2000+ will outperform an Intel Pentium® 4 processor operating at 2.0GHz on a broad array of application categories.