The is a difference, how ever it is only a clock speed as they are the same chip with a different clock speed.
E6600 Vr E6700
9% Performance increase (clock speed increase)
62% more expensive ($369.99 Vr 599.99)
So....... Price > Performance by almost 7 fold.
Not worth it.
I am just basicaly saying what other have already told you
overclocking no... not overclocking, 270mhz or so... architecture, none at all.
price mostly if you ask me.