An educated homeowner is more likely to make the best decision.
Here are few great resources
A slightly cumbersome website with great technical data for every single replacement window sold on the market. Be aware that many companies “rebrand” their windows by giving it a different name. Ask for the name the manufacturer give the window and look that up on search.NFRC.org
An active site that was created by individuals interested in creating a place for consumers and professionals to visit and interact in the hopes that information could be shared.
A great resource put out in partnership with the Lawrence Berkley Lab in California. Easy to understand information, state guides, case studies, and an impressive glossary of terms are all readily available.
An active online discussion forum. Look at what other people have asked or put up a question of your own and get feedback from homeowners and window professionals alike.