Anyhow, after having read reviews of the Coppermine 500 like this one at 3DHardware, I'm convinced. By spec, the PIII 550E runs at a 5.5 multiplier on a 100MHz bus. However, putting that puppy on a 133MHz bus makes for 733MHz of overclocking fun. Thing is, people are running the thing at over 800MHz with some regularity now by raising bus speeds even higher.
The Athlon is still the most versatile overclocker on the market right now, in my book, but the integrated L2 cache of the 550E and the easy bus-speed overclocking (generally with only a BIOS menu settings change) make the 550E my pick for "the next Celeron 300A." The king is dead; long live the king!