There is a great two part article by Arrol Gellner at about the importance of maintaining the beauty and architecture of your home when replacing windows. The style (double hung, casement, etc) and the materials/design of your windows is vital in maintaining the value of your home.

Integrity - Wood Ultrex
Part 1:

The second part of this article discusses the importance of window materials (vinyl, aluminum, wood) in maintaining your homes value and beauty.

Part 2: