Thanks much to everyone for replying.

the load calculation: Both guys measured the furnace and basement
ductwork. The Trane guy measured all rooms individually and did look
into the crawl space at the insulation; the Bryant guy took an
aggregate measurement of the footprint both upstairs and downstairs.
They both appeared to be doing lots of math; whether that math
resulted in the load calculation is not clear to me. I will find out.

(As an aside - the first quote I got was from a guy who walked in,
wrote down the numbers from the existing furnace, said "I'll get back
to you," and left. His quote was cheapest of all, but I ruled him out
based mostly on the fact that I couldn't even get him to stop to talk
with me about the job... although I did ask the question about getting
the AC upstairs as he was backing out the door. He basically said, "We
can't do anything about that." Sheesh...)

Very good points the thermostat - thanks. I'd hate to invalidate
the warranty or service contract over a few bucks. (BTW - Travis, you
are correct. I was looking at single stage.) It might be worth noting
that the Trane guy included a programmable thermostat (also Trane
brand) in his quote at no additional cost. He also will also reinstall
my existing humidifier (an Aprilaire 550, I think); the other
gentleman proposes installing a new Honeywell electronic air cleaner
for an additional fee. I don't know anything about "air cleaners," so
I'm not sure whether it's worth going that route or not.

Regarding the returns: I know that they exist upstairs - they are
positioned exactly opposite the supply registers in each room - but
that's all I know. HVACmedic - if I'm reading the link you provided
correctly, I have a "trunk and branch" config in my house.

So - I will get a few more quotes, and will be able to ask more
intelligent questions as a result of your input. Thanks again for your
help.

Ali