I assume you are comparing the Pentium M and the Core Duo. Video playback decoding is generally not multithreaded, but the dual core chip will still win out due to the higher core frequency.
Either chip will likely handle standard 720x405 encodes with the ffdshow or CoreAVC decoder. If you start getting into more advanced encoding, for example with B-frames enabled, then you could run into some performance concerns, especially with the slower P-M chip.
Anyway, if high-def video is your goal, then you want to have the highest performance processor you can afford. However, also remember youf video card selection can play a huge role in your decision as well, as a poorly performing graphics accelerator is certainly not going to help you reach your goal. At minimum, I suggest looking for a graphics option with its own dedicated memory instead of relying solely on a shared memory architecture.