Well the E4500 has a higher multiplier than the E6400. I believe it's x9 or x10, whereas the E6400 has an X8 multiplier.
The most crucial difference is the cores themselves. The E6400 is a Conroe based core which is known to run cooler and over-clock significantly better.
Don't take that as the final straw because plenty of people have gotten E4xxx series to hit 3.4-3.6 GHZ, but the people that do by percentage aren't as high as the amount of people with Conroe based Core 2 Duo's.
The E4500 uses the Allendale core.