Very simple, go to service.madonion.com, and run some searches to see the scores between the P4+9600's Vs AMD+9800's.
Get the 9800 system. Even a P4 3.2EE overclocked to 4 GHz will have a tough time beating a moderately overclocked AMD with a 9800 Pro.
EDIT: I just searched myself, here are the results:
Top 3 scores with P4 and 9600:
18252
17721
17320
(Slowest P4 of the bunch was running at 4.1 GHz)
Top e scores with AMD XP and 9800:
(Filtered from 2.0GHz to 2.3 GHz since that's about the average OC on a Barton)
24923 @ 2200 MHz
24191 @ 2150 MHz
24102 @ 2200 MHz
All from unique users. I know these scores are extreme, but it just shows how much more of a potential the 9800 pro has over the 9600 series, uncluding the XT's.
The best score using AMD was 28197 running at 2821 MHz.
But not like you're gonna get your AMD to run that fast anyway, nor will you be using a P4 EE @ 4.1 GHz