Real basic comparison:
AMD XP procs does better at floating-point calculations. This means that you may see a better 3D experience overall in speed. AMD uses 3DNow!, MMX, and SSE instructions.
Intel P4 chips aren't very consistent. The newer Northwood chips have been given more L2 cache (512kb) which has significantly improved speeds in office apps and similar apps that involve educated guessing by the proc. Overall, the P4 beats the XP proc in terms of "chugging away" so to speak. Whenever the task is simple but involves large shunks of data (such as when encoding an MP3), you'll see better performance on the P4 due to its long pipeline.
I guess that wasn't very basic, but here's my point:
Both have their advantages and disadvantages. If your budget isn't small, I'd recommend one of the newer northwood P4's (2.0 or 2.2 GHz). If you want to concentrate on gaming and investing most of your money into sound and video, save some $ by getting an Athlon XP. Both are good procs...