My house is 70 years old with the original single-hung windows. I love them. They have four vertical panes on top and one single pane on the bottom. Vinyl replacement windows would just look terrible and really diminish the character of the house.

I plan on repairing my existing windows (like FlyingScot did) and putting up good-quality storm windows and screens. The storm windows are not all aluminum frames these days. They can be painted to match whatever color scheme you have on your house. With properly repaired windows (sash repaired, weatherstripping, storms), the savings is at least as good as the replacement windows, and there are no old windows to add to the landfill.

Thanks for the link, FlyingScot. I may not get the tax credit (but I think you still can for stormwindows), but for me, it's important to retain the character of my house while still increasing the energy savings.

While I have not looked into it, there might be replacement windows that are wood framed, copy historic patterns, and are super efficient. But you still have the problem of how to dispose of the old windows.