Took you up on your challenge and I did research it further, here's
what I came up with

http://www.nfrc.org/press/morewindowfacts.aspx
"Household energy costs in most U.S. homes could be reduced by up to 15
percent by installing more energy-efficient windows."
that's UP TO
Here's something specific:
http://www.energystar.gov/index.cfm?...pr_savemoney#1
"Estimated savings for a typical home from replacing single pane with
ENERGY STAR qualified windows are significant in all regions of the
country, ranging from $125 to $340 a year."

So I'm paying about $2500/year for gas heat in the winter and Electric
A/C in the summer. So if I replace 27 windows in my house at an
approximate cost of $800 each or total of about $20, 000 it'll take 66
years before I break even, if I save $300/year. OK so there are many
other factors that come into play here, but the gist of it is that from
a financial perspective you'll realize a small drop in utilities but it
will take a long time to actually see a Return on Investment from
replacing all your window.
I'd still advocate stripping each window and renovating them, you'll
remove the lead, add weatherstipping and improving their quality so
they last another 100+ years.