It all depends on what you plan on doing with your system. If you know you are going to use apps that take advantage of hyperthreading, the p4 will be better even with a "lower" clock speed, but for everyday use, I'd take the Athlon.
The price gap has closed quite a bit between Intel and AMD.
p4 2.6c, Asus P4P800dx, 2x512mb PC3200 = $543
AMD 3000+, Asus A7N8Xdx, 2x512mb PC3200 = $568
But if you do a 3.0ghz p4 to 3000+ Athlon comparison, it is $568 for the Athlon and $643 for the P4.
Any way you go, the speed for the price is remarkable.