Both processors are Very good. When I first had the same choice, I went for a 3Ghz Amd 64. It was a nice, quick machine.
Two months later I had the chance to sell my Amd for the price I paid for it. I then went for the P4.
These are the main differnences.
My Raid array (hard disk transfer) increased in speed by 50%.
When multitasking, program run flawlessly. (on the AMD you get pauses and slow downs).
And if it's games your after, I can run farycry with everything on max inc 4xa at 1280x1240 and get an average of about 50 fps.
Good you might think, but this is while e-donkey is downloading and norton is doing a full virus scan in the background!
Norton alone used to kill my AMD, I used to leave the computer until it had finished.
I'm not against Amd's, I've used them for years but I wouldn't go back