Both Prescott and Northwood are introduced in 3.40 GHz versions today, and both are packaged in the current Socket-478 platform. 3.40 GHz will be the final stop for the Socket-478 platform at the high-end, however.
When Intel introduces a 3.60 GHz variant of the Prescott processor in Q2 2004, it will be on the new Socket-775 platform only. Socket-775 boards will have much tougher power design specifications that will be necessary to feed these thirsty processors at 3.60 GHz and above. Subsequent versions of the Prescott processor, including the 3.80 GHz in Q3 2004, and the 4.00 GHz in Q4 2004, will appear on the Socket-775 platform only, as will Prescott’s successor, Tejas, in 2005. All Prescott Pentium 4 processors will operate with an 800 MHz FSB, and will feature Hyper-Threading Technology (excluding the 2.80A GHz model, which uses a 533 MHz bus and no Hyper-Threading).
At the low-end, Intel will continue to use the Socket-478 platform for its Celeron processor through 2004. Over the year, the Celeron will slowly ramp up to 3.33 GHz using the 90nm process, and continue to use the Socket-478 platform. It will eventually migrate to the Socket-775 platform as well near the end of the year. All 90nm Celerons will get a bump to 256kB of L2 cache.

Fig. 3 - Intel's current 2004 roadmaps suggest the above processors will be introduced in the timeframes indicated. The last Socket-478 Pentium 4 processor is the 3.40 GHz parts introduced today.
As an aside, these 90nm Celerons may be of some interest to overclockers. A 2.53 GHz (533 MHz FSB) Celeron using the 90nm process will be introduced in Q2 2004, and depending on its price and the maturity of the process at that point, may prove to be a capable overclocker.