ok i viewed some benchmarks about those 2 cpus,
there is actually just a little-bit of extra performance for the E7400 over the E6300, and that is not even noticeable.
the E7400 is a little faster Maybe by 5%, is because it have more Cache, just 1 extra mg
but keep in mind that E7400 lacks of IV (Intel virtualisation) thats all.
as in for differences between Dual core vs Core 2 duo, it all depends on which ones your getting, as in some Dual cores are faster than Core 2 duo,
because they both have 2 cores
they both don't have hyper threading
alot of them have the same FSB
they both use intel 64
so basically alot of them are in common, it all matters on which one you chose as there is no real difference between them both.
hope that helps.