Because windows are so expensive and you expect them to be in your home for decades, do a lot of reading and research. I would start with the NFRC website. I learned a lot there. Then I would hang out on this Windows Forum, do searches of old threads and read all you can. Then find a good contractor--not the cheapest one--that gets good ratings on Angie's List.

http://www.nfrc.org/WindowRatings/

look at these 3 things-

a high quality window, particularly high end vinyl, will have the following attributes.

Total Ufactor of .28 or lower.

air infiltration rating- of .06 or lower.

Structural rating- of 45 or higher.

finally, a window should have a true welded sloped sill. not a pocket sill and not a snap in sloped sill.