If you're in the market for some weatherization, you have a lot of options on where to spend your money. You can get new vinyl windows, insulation, or a new furnace. If you happen to able to afford all three this article is not for you, and, umm, share the wealth. But for those who want to save money and the planet, getting a better furnace is your best option.
In the 6 months we've been here, we've learned that new windows, top notch insulation in the attic and below floors will save you money. But not as much as having a 90%+ efficient furnace. We have an 80% 3 year old one that works well. But we can tell a big difference from the awesome 2 stage 90% we put in the condo last year. Our most expensive month was $50 and that was keeping it at 70 degrees because of the new baby. Keeping it at 67 here has run us more than that and it cycles off and on a lot. So if you think 80% will be fine and you'll put the rest of your money into windows or insulation, don't. Buy the 90%.