There is more to the Thunderbird "C" model than just the 266 factory unlocked clock speed. From what I understand:

It is true that you can unlock your Bird or Duron by connecting the L1 bridges on the CPU. This will change the speed of your Front Side Bus. However, there is another bus that runs from the CPU to the memory or DIMM slots. Because of this, you will need a C model T-bird (266MHz) to run PC2100 memory if you get a DDR board. If not, you can only run PC1600 memory.

I haven't yet heard of anyone using a regular T-bird in DDR motherboard with PC2100 DDR RAM and now I'm wondering if some Dresden plant Blue 'Birds might be good enough sit with PC2100.