Not an expert, but I did replace wooden framed single glazing in a tenement flat with UPVC double glazing. Made a big difference in terms of keeping the flat warm, definitely used the fire and heating less. Biggest difference was in the sound levels from the traffic outside. The advice I got from a friend who worked in an estate agents at the time was that new windows wouldn't add much in terms of value to a flat, but they would make it more sellable.