OHJ had a great article in their Sept/Oct 2007 issue, "Embracing Energy Efficiency." Their graphic showed a single-pane window coupled with a storm window with a combined u-value of 0.5, better than the double-pane thermal replacement... not to mention $400 cheaper!* Yes, a low-e double-pane window has a lower u-value at 0.35, but the payback is considerable... at 34 years, probably longer than the window itself will last, and very likely longer than your warranty on a new window. So instead of putting one window in the dumpter--the original, replaced sash--you'll be putting a window in the dumpter every 20 years! And paying $550 for the low-e opportunity!
* u-values rate energy efficiency. We found this article from doityourself.com helpful.
The greenest building keeps its windows out of the dumpster!