When you start to look for new windows for your home, you won't want to skimp out on quality. The main reason for this is because lower quality windows will actually end up costing you more money in the long run from the lost heat that will escape. You will want to make sure that whatever window you choose has a good Energy Rating.
This rating will take into account how large or small the window it, the performance and quality of the glazing, frame construction, and spacers. The rating will be written as a number that's based on the amount of air leakage in and out of the window. If that number is negative you will actually losing more heat than you gain. If you find a window with a positive number it will actually assist in the heating of the home. Generally this will be done using great frame design and using solar energy.
Look for low-emissive or argon gas
There are double glazed windows out there that are considered low-emissivity windows. Some of these windows will even come with argon gas trapped inside. These types of windows always have a rating right around -9 and above. The reason for this is because the argon gas in the inside actually stops some of the air from coming in, and it will also block much of the UV rays that could be damaging your curtains or furniture and art in the room.