Athlon's hold the best-bang for the buck.
You can get a 1.8Ghz Athlon XP for about $61 and a Nforce2 motherboard for as low as $59.
Meanwhile a P4 1.8Ghz CPU is $122 and a i865 based motherboard is $78.
Keep in mind the P4 is going to be slower than the Athlon dude to their internal differences. You'd actually be looking at having to get a 2 - 2.26Ghz CPU to match the 1.8Ghz Athlon.
Now unless you have $655 for a P4 3.2Ghz, $125 for a i875 motherboard and the ability to chuck your current DDR for DDR400 then I would not go intel
