So you should save the money and go straight for the 6600. The only real difference between the 6600 and 6700 is clock speed... and the q6600 is easily over clocked.
I've read that the Q6600 built to the "SLACR" Stepping, 95W-thermal-design-power is the variant to look out for. That spec is on the retail box itself.
Of course you can run it at stock levels for a month or more and then turn it up after it has all soaked itself in nicely. It isn't uncommon to be able to hit even higher rates months later. My experience has been that the RAM and RAM controller (in the northbridge) is what improves the most over time.
The question for me is whether I'll be able to use the extra power. Then again, I'm rendering overnight typically and I find that the numbers of layers of HDV I'm playing back on a far cheaper Core2 Duo is enough for me. Always good to have more and I'd have probably had paid for the Q6600 if it had been available when I ordered my components.
I'll probably buy again if CPU grunt is the only way to overcome H.264/AVCHD editing performance.
3GHz is readily achievable and my usual guide for a safe clock is to choose the maximum speed available in that model of CPU (using the S-Spec as your reference). The Extreme processors maybe cherry picked from the batch but I think this isn't something to be too hung up over.
Watch the temperatures initially. The Intel CPUs have temperature protection on them in case you push them too far. It is bad HDD writes and filt integrity issues that are probably your largest enemy if you start trying to push your CPU or RAM over the 3.4GHz mark. Hence why I posted the above link to a company who has evaluated the risk:reward for you. Check such dealers' small print but they are removing a lot of the heartache out of enjoying the cheaper speed chips at their expensive equivalents speeds.
"How far can these chips be pushed? What would the "safe" overclock speed be for the Q6600?"
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I've got mine sitting at 3Ghz... could probably go higher if I invested in better memory... but 3Ghz is plenty fast enough for me!