Well to perfectly honest it really depends on how high end you watn to get with your system.
If you are going all out balls to the wall then the top of the line P4 is what you want. The 2.8 P4 is smoking the high end Atholons of the present in most benchmarks (source is the latest issue of Maximum PC). If you are not going all out top of the line then I would say go for the AMD chip as they are way more bang for the buck. The P4's will outdo them in some categories, but the cost difference can be pretty substaintial and can outweigh the slight performance edge.
Just my 2 cents worth.