price verses performance; AMD wins with almost all its processors (except the FX's)
all of AMD's processors perform much better clock for clock than P4's, and especially for how much you pay for them
Athlon XP 1.67GHZ processors performs about the same as P4 2.5GHZ processors, which is a 150% performance increase clock for clock
Intel has a 64-bit processor called the Itanium, but it is very hard to get hold of; is Intel ashamed of it?