BCC Hardware posted a comparison review between Pentium D vs. Core 2 Duo. To be more exact, they have put the brand new Core 2 Duo E6600 at 2.4GHz against the 2.66GHz Pentium D 805. Here's an excerpt:

Yes, the Core 2 Duo E6600 is faster than the Pentium D 805. Are we surprised by the winner? Of course not, but the margin of victory was larger in some cases than we had ever expected. On the other hand, there were some instances where we expected more performance out of Intel's latest architecture. We saw that even though the Core 2 Duo is clocked slower than the Pentium D 805, the amount of work it can do per clock cycle is much, much higher. No offence to Intel fan-boys, but it's about time that Intel made a decent chip that wasn't sitting spinning it tires in the mud. It's great to see the massive performance improvements that are available - not by clocking a processor faster - but by making it work properly at the speed its running.