I find the whole thing a bit silly. In the end the point is to save money and get the same performance as a high end product without having to spend as much right?
Well the E6600 costs $320
The E6400 costs $220
Typically the people who buy the E6400 boast about having over clocked their CPU to the same speeds as the E6600, and saved about $100.
Which is fine, and good, except not every motherboard will work well with over clocking. While a E6600 will run fine at stock speeds on any mobo, the E6400 needs a more 'elite' mobo to handle the over clocking. I noticed that mostly the costs of such a mobo are about $50-100 more, and if you count in a $50 Zalman fan then is any of this really worth it?
In the end there is no guarantee with over clocking either. I think over clocking is an important art, but I think in most cases it is only worth it when your hardware is behind the times... not fresh out of the box!