In retrospect, it depends on the specific test. Id imagine that anything involving folding or The GPU the C2D should outright win. But prehpaps in SuperPI the Pentium @ 8 Ghz May beat out the C2D. I doubt will will ever know.
E4300 1.8GHz beats Pentium D 960 3.6GHz in most cases
1.8 is half of 3.6
4 is half of 8
So, E6600 @4.0GHz should beat Pentium4 @8.0GHz in almost everything