I might be wrong but i seem to recall that HD footage would only play back on machines with Dual 2.0ghx G5 processors or above, or mac's with intel processors. I know that my iMac single G5 2.0ghz would still stutter with HD video.
I have an intel MBP from 2 years ago, HD works perfect, so you shouldnt have any problem with the current range. Depends what your gonna use your MBP for as to whether its worth paying extra for processor upgrades. . . . . at the end of the day faster is better.
For reference i use my MBP to render 3D visuals and animations in cinema 4D, although not the quickest machine, it certainly doesnt struggle with it. My spec is: Intel Dual Core 2.4ghz, 2GB RAM, 256MB Graphics, 15" screen.