You can add an electric heat pump to your current system and use what is called a "hybrid" system. The heat pump will heat your home until it gets close to freezing outside, then the gas furnace will kick on and heat your home during those really cold times. It is about the most efficient system available, besides a ductless system. It's hard to estimate the cost of a heat pump as every house is different, but it's probably in the $7,000 to $9,000 range. The heat pump unit has to go outside and will connect to your current air handler and furnace inside, assuming that your current air handler and furnace are in good shape. Hope this helps, let me know if you have any follow up questions. Thanks.

Gordon Smith
Revolution Green Power