Oh jeez, global warming
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OP, if you have natural gas, hands down it is the cheapest way to go and should be that way for the long term. That's what I would do in your shoes, it will pay back the quickest and be cheap and reliable for a long time and you can probably use your existing ducts. If that doesn't work, replace the AC unit with a heat pump, use your existing duct work and I think you will be very happy with that.