Paper thickness (weight) to use for card tabs/dividers?
The main consideration in my experience (many, many Magic cards) is that the thickness/weight of the paper doesn't matter as much as the stiffness and the smoothness of it.  you'll want something that slides out easily, so the cards can be retrieved without losing the divider in the box.  The stiffness is important because anything too flexible will tend to fray the tab portion.  Since dominion is a standard card size, I'd highly suggest plastic dividers, which can be obtained for as little as $0.10 each at various stores.