There's a lot of benchmarks showing Northwood faster than Prescott, but those articles were written at a time when the Prescott was brand new. The results might be different with newer applications, especially if they utilize SSE3. I'm curious about that myself. Gaming-wise, I wonder if later games like the Prescott better than the games of ~2004 did.
I'm pretty convinced that the Prescott is better than the Northwood for decoding H.264 video, at least in VLC player anyway. It matters because those videos are marginal on a P4. The Prescott can manage to play things smoothly that will stutter on a Northwood. Maybe it's the cache, but I think it's more likely that the reason is the player/codec/whatever is programmed to benefit from SSE3.
I've tried 3 CPUs for H.264 playback on a Dell GX270 (865G based):
Northwood 2.6/800 Hyperthreaded - dropped a lot of frames on more demanding videos. Runs cool.
Prescott 2.8/533 non-HT D0 stepping - dropped a bit less. Runs hot.
Prescott 3.0/800 HT E1 stepping - Major improvement, almost perfect. The only way I got it to drop frames was with a high bitrate, fast moving 1080p video with the H.264 deblocking filter enabled. Runs noticeably cooler than the D0 Prescott did.
If you want a P4 to be able to play H.264 video, then I think the combination of Prescott + Hyperthreading is what it needs.
I'd still like to try a 3.4GHz Northwood HT, but they're a little pricey for something I might not end up using. Using the 2.6/800 as a guideline, I don't think it will scale to the same results as the 3.0 Prescott in video playback. Other applications might still prefer the Northwood though.