The answer to your question comes down to IPC (Instructions per Clock). When we first released the Intel® Core™ 2 Duo processors I was able to go and check the performance of the Intel Core 2 Duo E6400 2.13GHz and compare it to the fast of the older Intel Pentium® D Extreme Edition 965 3.73GHz processors in a number of benchmarks. Of those benchmarks the Intel Core 2 Duo E6400 outperformed the old Intel Pentium D in 16 of 20 benchmarks here on Tom's and only just lost in the other 4. The reason why is the great increase in the performance of the IPC on the Intel Core 2 Duo generation of processors and the following processors have only gotten better.
Christian Wood
Intel Enthusiast Team