Recently I have upgraded from a P4 3Ghz (S775) to a Core 2 Duo E6550 (2x 2.33GHz with 4MB cache). The difference in performance is astounding. I’ve not managed to find anything yet that will slow it down to cause any noticable problems with performance.
The cores also run incredibly cool. Before the upgrade, my P4 ran at between 50 and 60 decrees C with quite a substantial heatpipe heatsink + fan. With the stock cooler, each core of the Duo runs at between 30 and 40 degrees. Couple this with a stock cooler that supports PWM (and a mobo that does the same) and I’ve ended up with a much quieter and cooler system than before.
I can’t compare to AMD’s offerings cos the only AMD machine I have is a Duron 800, but you currently have a P4 I highly recommend the upgrade. You may, as I had to, need to upgrade your motherboard too, but I needed to do that anyway.