Historic home owners will often stick with their beautiful old inefficient windows simply because the modern replacements are so lacking in character. You can't get much closer to period-appropriate than Andersen Windows' 400 Series windows, like the new diamond panes here. They match the sash to the surrounding frame, and also avoid the telltale plastic "hardware" with a wide range of metal options (INSET).
NextNeed More Info? Ask a question on Home & Real Estate message board