Use this calculator to estimate the cost saving and greenhouse gas reduction for upgrading your insulation or windows.
You may be surprised how quickly more insulation will pay for itself.
A caution: This calculator is likely to overestimate the savings you will get for adding insulation. The reason is that it does not take into account that part of the heat for your house is provided by warm bodies, electrical appliances and lights that generate heat, and solar gains through windows (if any). The amount of the overestimation depends on your climate, your house, how you live, how efficient your appliances are, etc. Climate is the biggest factor -- for cold climates the, the amount of the overestimation will likely be small, and can probably be ignored. For warmer climates (say 3000 deg-days and less), the error can be considerable, but in these cases, you will probably also have cooling costs that the insulation will help reduce.
So, like most engineering tools, its a useful guide, but not infallible.