I wouldn't say that 2000mhz is the right speed...i have a 2500+ with 333fsb and its only 1.83ghz...
Some speed differences aside:
2600+ with 266 MHz FSB: 2.13 GHz
2600+ with 333 MHz FSB: 2.08 GHz
With the release of the Barton core, much of the clock frequencies were lowered, however, the extra cache and such helped the boost of the rated performance.
Furthermore, the Athlon XP 3000+ with the Barton core runs at the exact same frequency as the 2700+ with the Thoroughbred-B core (2.17 GHz), just only faster because of architectural improvements.