I have the exact same thread open!
It seems the best Q6600 clocks to 3.4, with an average of 3ghz (I wrongly quoted the QX6700 no's in my thread).
The e6850 seem to be hitting 4ghz on air...
So it depends what you'd rather have, 4 cores on lower clocks or 2 cores blistering fast...
Personally, I decided on the e6850, a move to quad possibly @ penryn, deffo nehlam.