My old one-story home has an old inefficient gas furnace. I would like to have a new, high efficiency furnace put in before winter. Like many people I like to research these things before spending a lot of money. So here's my current question:

I used the calculator on the site listed below to estimate the Btu requirements for the upstairs rooms in my house. Based on that calculation my upstairs requires a total of 38,000 Btu. My current furnace has 40,000 Btu output. I would also like to heat my finished basement, but am unsure how to add that load into this calculation. I understand that the natural insulation of the earth plays a big role in the heating and cooling of the basement, but I am unsure of how to figure that into the furnace sizing calculation. Based on the numbers above I would assume that 40,000 Btu is a little on the small side for my application. Is that true? I am looking for some direction on this and any feedback on the numbers I mentioned above.
http://64.226.150.132/hydronic/hea/flashpro.html (Thanks HKestenholz)

As always, thanks for the help.