Hi a.rowe55
It all depends what you are using the processor for. The Celeron is a cut down version of the Pentium 4, but for anything except the most processor-intensive tasks (such as games and video editing) it will be more than adequate.
In fact as long as you have good supporting components, such as lots (>256Meg) memory, a fast hard drive and a good graphics accelerator card, a Celeron 2.6GHz is more than adequate for most applications.
I have a 1.4GHz Athlon in my main PC, a 800MHz Duron in my son's PC and a Pentium 4 2.4GHz in my laptop. They all appear just about as fast as each other for all the applications I use