The e6600 overclocks just as well as the rest of the C2d's. I don't see a reason to get a e6600 when you can get a e6320 for less and just oc it.
The E6600 has a higher multiplier so it should overclock farther then the E6320.
A higher multiplier makes it easier to overclock, it doesn't necessarily make it more capable. The C2D's all use the same core.. other than L2 cache, a higher FSB,Multiplier they are all the same chip. The only way a higher multiplier will help is when you're motherboard doesn't support a high FSB bus speed. So the only logical (spockified) reason to get an E6600 is if you aren't going to overclock, which most people don't do.