When buying a Q6600 you should use it with a board that can reduce the VCore in idle-mode, even when overclocked. As far as we know, only MSI's P35 Platinum can perform such a "miracle." All other boards won't reduce the VCore while the CPU is overclocked, which results in much higher power consuption. In a non-overclocked enviroment the Q6600 stays closer to the dual-core counterparts; when overclocked it uses 10-15W more than our old E6700 dual-core, an E6750 would increase the difference by another 14W.
Under load we see similar results as we did with the QX6850. While not or slightly overclocked the power-consumption is not that far off from an dual-core; the 3330MHz settings take another 40W.