Last Monday we got new windows installed in our house. We wanted to keep them looking like the original historic windows, but needed something better to keep the winter wind & cold out. We got energy efficient low-e argon vinyl windows with muntins inside the double panes to keep the historic look.
They have a .30 U-factor, which determines the rate of heat loss (here in our climate .35 or less is recommended) They have a .19 solar heat gain coefficient (the fraction of solar radiation admitted through a window, expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.).
Here are some pictures:
The same window - the first picture is the old, the second is the new. This one's in our dining room.