Anandtech didn't compare an overclocked e6600 to a quad core, but they did compare an overclocked e6300/e6400 and an overclocked e4300 (the cheapest core 2 dou you can buy) to the other core 2 dous.
AnandTech: Intel Core 2 Duo E4300: Affordable and Highly Overclockable
AnandTech: Intel Core 2 Duo E6300 & E6400: Tremendous Value Through Overclocking
This article shows you how far the e6600 can go with overclocking on air:
AnandTech: Intel's Core 2 Extreme & Core 2 Duo: The Empire Strikes Back
They get it to 4 ghz stable but i doubt you would want to leave it that high (temps are prob high) on air. 3.4 ghz (what ive got myn at on a zalman 9500) is sitll fast. So my e6600 will beat a stock x6800 since they both have the same specs, but my e6600 is overclocked to 3.4ghz and the x6800 is at 2.9 ghz. Now the x6800 can overclock really well with the unlocked multiplier, but its still as expensive as the quad cores. So you might as well go with the qx6700 if you consider the x6800.
The e6700 only has an extra multiplier over the e6600 which may get you a little higher in overclocking but not the much. I actually use one less multiplier on my e6600 so i could get more fsb out of my ram...8x425 instead of 9x377 (so the e6700 would have been a complete waste of money for me). So again we're back to the " is the extra few seconds less in encoding or few frames more in gaming" worth $200.
All in all the e6600 is the best cpu for price and overclocking. If you were on a budget (which your not since you were considering the quad core), the e6300 or the even the e4300 are great cpu's that have decent overclocking potential. The only thing you lose with the lower end core 2 dou's is the extra 2mb cache. But again your not on a budget so the e6600 is perfect for you i think.
As for a motherboard, if you want sli then the 680i boards are your best choice. The 650i boards are the budget sli boards that will run sli at x8 x8 speeds compared to the 680i's full x16 x16 speeds. If you don't plan on sli'ing, then the 965p chipsets are awesome, rocksolid mobos.
Good 680i boards are the evga 680i or the asus p5n32-e (the striker is just too expensive for some extra gadgets and asus's cool name tag
Good 965p boards are the asus p5b-deluxe (the best hands down my opinion
) or the gigabyte ds3. Go with the p5b-deluxe though. I have had zero problems with myn and it is very easy to overclock with this mobo. The ds3 has great potential for the price but as i understand it, it takes some fiddling with it to bring out that potential
Hope that helps.....man i wrote a long post