My reccomendation for a good AMD CPU is a "Barton" 2500. They are fast and stable, even after some Over Clocking. I am running 2 machines that both have a barton, and have had no problems. A good place to find a good price on AMD CPU's is
www.newegg.com They have helped me out alot
Truckie
PS: And yes a "Barton" is faster than the P4 you are using.