Replacing the windows is the easy part, choosing the right window may be the hardest part. Wood windows, vinyl windows, or even fiberglass, which is the best, how do they compare?
Most all window companies will offer standard high performance glass packages which will include low E coatings, and argon gas. Make sure you check out the "U"value (lower the better), the Visible light transmittance, and the Condensation rating on all windows you are considering. All of our Marvin, Pella, and Richlin windows have certification stickers on every window.
Make sure you understand how your windows are going to be installed. Are the existing windows being removed completely down to the studs or are you getting insert or pocket windows where the new windows is to be installed within the existing window frame leaving all of the interior woodwork as is. Typically full replacement will give you more glass, insert or pocket installs tend to shrink the window size because you are installing a window inside of an existing frame.
Are you getting a maintenance free exterior? New aluminum trim or some type of composite material may be what you are looking for.
All new windows need to be shimmed, insulated with low expansion foam, sealed, and secured into place.
Custom sizing, made to order, is now available from most manufacturers. This will help to ensure you get the proper fit and finish.