Definitely see where you can insulate better or replace windows if needed. That'd be my first step.
If and when you do replace your HVAC unit. Replace it with a dual fuel system. It is a heat pump with gas back up heat. Best of both worlds. You have the efficiency of a heat pump down to around 35* with out drying out the air like the gas heat will do. You can then set a lock out point if an outdoor sensor is installed and lock out the heat pump and only run gas heat below 35* or what ever set point you decide on. It also works as a typical heat pump, when it goes into defrost instead of bringing on expensive resistance heat, it'll bring on the gas heat during defrost mode. Of course, this unit is slightly more expensive. However, you would make back the difference fairly quick. Also the efficiency of the new system would outright save you more than that older system your running. Heck may save you enough per month to cover the monthly payment if you financed it or had a mo pmt on it.