Same difference as a P3 vs P3B
Faster front side bus.
The older TBirds use a 100Mhz DDR (200Mhz effective) FSB, the newer ones use a 133Mhz DDR (266Mhz effective) FSB.
The "266"Mhz FSB TBirds are quite a bit faster at the same clock speed than the "200"Mhz FSB Versions.
But you need a chipset to support it, KT133 doesn't. KT133A does, and any of the DDR-SDRAM chipsets do.