Windows 7 Custom Autosnapping sizes

When I press Win-Left/Right, the current window snaps to the edge of the screen and is resized to half a screen.

I want to customize the feature: i.e. I want snapping to the left make a window two thirds of the screen and snapping to the right one third.

Is that possible? Google found nothing on the topic. I couldn't see such a key in regedit.