The newer generation of processors is more efficient at the same clock speed than the previous generation. About 10 years ago, AMD processors were more efficient than Intel at the same clock speed. I understand that Intel rectified the situation with the core 2 processors, which tells me that they are likely very much more efficient than previous generation processors.
I had an older Pentium laptop at 1.6GHz, and a newer ultra low voltage core 2 duo laptop at 1.5GHz. The newer laptop runs SuperPi in about 2/3 the time of the older laptop.
I have a Core 2 Quad desktop overclocked to 3.4GHz. I've built some new i5-750 desktops recently and at similar 3.4GHz they seem to be about 15% faster.
So just running a single app, I'm guessing that the core2 duo will be a good amount faster than the Pentium. PLUS a dual core processor has a lot of real world advantages over a single core processor. I mentioned the dual core low voltage laptop above. I've wanted a small portable machine ever since I first started seeing 9" and 10" netbooks, but I wasn't keen on a single core processor. When the ULV notebooks came out with dual core processors, I got one. I can run multiple things on my dual core desktop without them interfering with each other very much. I can run a full virus scan on my quad core desktop while I'm running other apps and never know the virus scan is running.