Just to inform you, the AMD64 3200+ doesn't run at 3.2Ghz - It runs at 2.0Ghz (Assuming it is a skt939). The AMD is faster, but don't be suprised when you see it runs at 2.0Ghz 8)
Which is why you should go with AMD because it can keep up with Intel at quite lower speeds.
But seriously, the AMD64 is what i would go with. I haven't exactly heard anything bad about either, but doesn't the P4 run at (a maximum of) 32 bit?
If thats the case, the AMD has the advantage because if any kind of big 64 bit software comes out that drastically improves performace, you won't be totally left in the dust