Celerons do have their applications, and not just for Joe Sixers. For example, they have relatively low heat output for their speed but they perform better than the Via Epia's, which makes them a good choice for quiet applications.
As I recall, the P4 celeron has 128 KB of L2, vs. 512 KB of L2 for the P4 Northwood. Now, if you were to take a Celeron and P4 at the same clock speed (with comparable bus speeds on identical architecture for the best comparison), you'd find that a doubling of the L2 cache gets you a 6-8% performance boost. So, let's apply this twice.
1.08 * 1.08 = 1.167,
so the quadrupled L2 cache gets a P4 a 17% performance boost, given an identical clock speed, bus speed, etc. Of course, it will vary according to the application.
Now, let's compare a Celeron at 3.0 GHz to a P4. (And we'll ignore bus speeds, etc.) 3.0 GHz / 1.17 = 2.56 GHz, so by this analysis alone, your Celeron should perform close to a 2.53 GHz P4. The additional .16 GHz = 160 MHz discrepancy is small, and any one of a number of things could account for it. (Differing mobos, memory timings, memory architecture, hyperthreading (I haven't kept up with which models of P4 have it at the moment. Feel free to chime in here.))
Just a bit of counterpoint.
-- Paul