AMD's do more work per clock cycle, which means they can perform as well as P4's at lower frequencies. There is much more to performance than MHz/GHz, however it seems intel effective marketing over the last 10years has clearly worked.
(Hence why the two companies are no-longer naming there chips by clock speed ' Intel 350 ' is an example of the new naming system)
As for am AMD64 the chip archticture, is improved over AMD XP chip's and generally is considered to be much more productive in number crunching and games.
This is a Masive question with many different view points, I suggest a Google search AMD64 vs P4 to answer your question more fully.