Me again ;-)
It won't skip playing MP3s, but I imagine that even an overclocked 400MHz PXA chip machine will drop frames depending on the encoding (and w x h size) of the movie.
This is more the question which you need answering - what are the best encodings for the two machines, is there much image/sound quality difference, etc.
There was a thread on the forum pointing to someone's website, where they had listed, quite extensively, how to properly encode videos for the Z. It might be worth tracking this down (it may also be in the HowTos section actually).
Si