This might be late, and redundant as well, but I also prefer AMD in general. Like thirdofsix my main machine is a Phenom II quad-core running at 3.4ghz per core, and it's blazing fast. I'm still happy with the performance of my Athlon 64 dual-core running at 2.4ghz clock speed, too.
That said, I used to do computer servicing and refurbishing, and there is a huge variety in the P4 line to the point that some versions of it might as well be different processors entirely. The difference between socket 478 P4s and socket 775 P4s is comparable to the difference between socket 754 and socket AM2 AMD chips.
A hyperthreading P4 at 3.0ghz is a pretty decent processor. A non-hyperthreading P4 at 2.5ghz really isn't. A hyperthreading P4 socket 775 clocked over 3ghz is going to still deliver pretty decent performance. The Pentium D and Pentium Dual-core (yes, they are actually different) can both deliver good performance. Heat is a primary concern with almost all of them, though.
As a rule of thumb I'd check the specs on a given P4, determine the socket, and determine the highest P4 that the motherboard could run. It might be possible to get a decent machine just by swapping the CPU with the fastest it can take, and P4 CPUs can usually be acquired pretty cheaply.