The only reason why I use a P4 (Northwood, not Prescott) is the i850E chipset, which is incredibly stable and compatible. Unforunately, AMD have basically stopped making their own chipsets, and I've been burned by VIA way too often. I won't trust them again, ever.
Right now, I'm using a 2.4 GHz P4 and I'm happy with it, performance-wise. I don't see the need to upgrade anytime soon. But if I had to, I wouldn't know what to buy right now... I'd probably try something nForce-based.