I am a great fan of energy efficient construction, and have purchased some of the most efficient glazing systems for my home and cabin, but retro-fitting windows is a very expensive proposition, and unlikely to create an investment return equal to a passbook savings account rate.
- Installing energy efficient windows in new construction has a much lower marginal cost and makes a lot of sense. (It is also most likely required by your local code).
Unlike cars, furnaces, and appliances, windows (and other insulation) can last for decades, and thus providing for long term energy savings.
Replacing those old windows has some other advantages: the new window will actually open and close; they may look better; they can help with soundproofing; and they will reduce interior convection currents. See the topic page on Preparing for Cold Weather.
The bottom line: New windows may have all sorts of advantages but they are unlikely to "pay for themselves".
See: A Field Guide To Bad Home Repair and Remodeling Contracts