I can vouch for what the others said from first hand experience. I have a Pentium 4 3.0 GHz in my the desktop I used until this summer and a new 1.3 GHz single core processor which is essentially a Core 2 Duo minus a core and they are comparable in CPU performance.
Everything else in the computer will also be a lot better too. Everything in my old computer was also high end for the time and my laptop with the 1.3 GHz single core runs significantly faster in every way.
So if you're asking this because you're hesitating on buying what you think is a slower CPU, hurry out and buy that Core 2 Duo.