My wife & I purchased a legendary 'Vancouver Special' (picture of house as we bought it attached). The single pane windows and 37 year old furnace needed replacing. As well, the attic needed more insulation. We chose a high-efficiency furnace and windows with Low e coating and Argon fill (doubling the insulation value of the windows). The furnace is 94% efficient; we chose the option with a variable speed fan. The indoor air quality is superior due to the constant air exchange. We added R-28 worth of insulation to the attic. We could have chose a less efficient furnace and avoided window extras as well as the insulation. The added cost was about $4000 for all these extras. We made the decision because we see ourselves in this house for many years and expect to recoup the costs in 5-7 years. We also reduced our personal GHG emissions by about about 40% (from space heating) we've concluded. We highly recommend adding on these extras if you can pull it off; we have no regrets!