Considering the recent price drops, that is a harder decision to make now than it was 6 months ago when I bought my parts. If you add the "extra" $100 to the cost of the entire system, your decision gets harder.
Do you put the extra money into an E6700, or do you put it into more, better, or faster RAM, bigger or faster hard drives, or a better video card? Do you add a PCI audio card instead of using the on board audio?
Consider that $50 will get you an HSF that will probably let you push an E6600 farther than an E6700 with the stock HSF if you overclock.
Consider your design goals for your system. What is the best use for the extra $100? That may include just keeping it in your pocket.
If you want a simple answer, the answer is , "Maybe."
Otherwise...decisions, descisions.