Thanks. I'm planning on getting a 90% efficient furnace. My real
question was about 2-stage and variable speed furnaces. Although the
more I read about it, I don't think the varaible speed furnace is the
silver bullet to my problems, especially with the age of my house and
less-than-perfect insulation/windows. I'll probably just go with a
standard 90%.

No one has done a load calc yet. They all say that before installing
they will do a load-calc to ensure the correct size, although everyone
has suggested 80,000 btu.