If you have just bought a new Q6600 then the cpu's that clock higher will be costing you more, so unless you have a genuine purpose for it stick with the Q6600 because it's great value right now, spend what you've saved on graphics or a monitor upgrade.
As far as certain q6600 OCing better then other there are 2 big things to look at. First is the revision. The older B3 do not OC higher then the newer G0 steppings. Second is the VID, from what i understand the lower the VID the better. Your VID looks to be around 1.2 which is ideal. Some chips will be closer to 1.3 to 1.4 which means there is less headway to up the core voltage when OCing.
Am I not understanding what I am reading in his picture for core speed? Is there something wrong here, should it not say 2400.xMHz instead of 1600.2MHz
^you are right on the first part but funny thing is when my CPU is idle its voltage is higher than when under load. This could of course be because of VDroop being higher when under load.