According to these roadmaps, 3.4GHz will be as fast as Prescott goes for Socket 478. Intel will transition to LGA-755 Prescotts in the second quarter, and chips are expected to scale to 4GHz by the end of the year. Interestingly enough, it looks like 2.8GHz Prescott chips will use a 533MHz front-side bus rather than the 800MHz bus shared by the rest of the Prescott and Pentium 4 "C" family.
On the Celeron front, 478-pin Prescotts will pop up in the second quarter with speeds between 2.53 and 3.06GHz. Prescott-based Celerons will use a 533MHz front-side bus and feature only 256KB of L2 cache, though the chips will scale 3.33GHz by year's end. Intel will also introduce LGA-755 Celerons, but those chips won't go beyond 3.33GHz this year.
The roadmaps also outline plans for Intel's upcoming Grantsdale and Alderwood chipsets, including the ICH6 south bridge.