personally, I'd go with the P4 and overclock to 4.6Ghz with watercooling. It's not hard. I don't care if you can overclock it that high or not, but I know that P4's have good overclocking potential, and even win over AMD's in overclocking benchmarks. My 3.0Ghz P4 benchmarked with SiSoft Sandra beats the 3.2Ghz provided in the charts. I don't know why, but it does, and if you want a screenshot, I will post one gladly, because I'd like to know why.
I'd also be happy to go with the AMD 3800.. its not a bad choice, but like everyone else says, you'd probably be better off with an Opteron