Performance wise, top end AthlonXP and P4 chips are on a similar footing.
However, and it's a big
however, there is a huge differential in price/performance between the two. Intel are still commanding a hefty premium for their top end chips, in addition the motherboards they are built on are often more expensive than their Athlon counterparts. It's much more cost effective to go AMD, if cost is important to you. More bang for your buck.
I'd recommend going for something just under the top end. An Athlon XP2700+ would set you back circa £270 inclusive whereas a more than respectable XP2200+ would set you back only about £120 - less than half the price.
A top end Pentium4 3Ghz is going to cost you about £550 inclusive, a respectable 2Ghz model about £140 (2.4Ghz for about £160).
Athlons also tend to be slightly quicker than their P4 counterparts with the same speed branding (ie. A Pentium 2Ghz would suffer against an AMD XP2000).
If you're buying a package system, someone like Mesh will probably provide you a better specified system built around an Athlon than someone like Dell could afford using a Pentium.
As you can probably tell, I've built a few systems now around Athlons and have had no problems whatsoever
