Advantages
 Enormous amount of savings can come from windows. If you have not upgraded, your windows will probably be a major source of heat loss. In cold weather, windows transfer heat out of the building through both conduction and radiation. Additionally, depending upon how weather-tight the frame and seals, windows may transfer energy by convection as well. This situation is reversed in hot climates, with windows allowing heat into a building and forcing expensive cooling systems to work overtime.
Typically, homes will have it's walls insulated with a value of R-11 to R-19, compared to an insulating value of about R-1 for single glazzed windows. So, heat can escaped from your home about 11 to 19 times through glass than through your wall.
Windows today use specially developed E-glass that is more effective at keeping control of heat and cold and where you want it. Most window manufacturers offer several lines of energy efficient glass with R values in excess of R-4. Newer designs still in testing stages promise R-values of 10 or more.
Since glass is a fixed part of the home, it performs 24 hours a day. With energy efficient glass, less fuel is required for a given level of comfort with accompanying cost savings.
Savings
Relating to the same or even greater level of comfort that you are used to, you can save a lot of energy and money by eliminating heat loss or gain through windows. It is common in Northern climates to save 30-40% of the yearly heating costs with efficient windows. A monthly heating bill of $200 dollars, you can realize a savings of $60-80 per month. Some large homes can cost as much as $600 per month to heat, and the savings for these homes could equate to $240 per month.
More Information On This Topic:
U.S. Department of Energy's Building Technologies Program: Windows, Doors, and Skylights
National Fenestration Rating Council (NFRC): Window Energy Performance Label
Energy Star Purchasing Tips
Energy Star Program Requirements for Windows
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