Given an equivalent clock speed, Athlon XP processors have, until recently outperformed their P4 counterparts. Hence, the re branding of the AMD product .. for example an XP2400 is not a 2.4ghz chip, but actually has a clock speed of about 1.7ghz (please anyone correct me if I've got that bit wrong) which, due to the performance, was marketed to compete with the P4 2.4ghz.
Recently, the latest chips from Intel have narrowed, if not eliminated the gap, and in fact recent reviews I have read now put the overall performance of the latest P4's ahead of Athlon XP.
Tim