We sell Republic windows in Iowa. We believe that for the money they are the best replacement window on the market today. Not just coming from a sales standpoint either goto Republicwindows.com and do some research. You will find that there are quite a few decision you'll have to make. I know, you can't get pricing off there site, however it will arm you with some good question to ask other dealers. Like what is the R-value of there glass packs or better yet what there u-rating is, the u rating is the inverse of a r-rating, loosely saying how much cold/heat does the product let through. Then goto the gov's energy site and see what is recommend in your area. Then there is VT (visible transmitel) rating. again loosely speaking, how clear is the glass or how much lite is allowed to pass through. Low-E is simply a coating applied to the inside side of the outer pane of glass in a double hung window, it is applied to reduce UV rays from entering your home. Low-E is double fold, it reduces the amount of fading of funiture and carpet plus helps stop the heat that is carried by the UV ray. Then theres argon gas, simply said "insulation". Argon gas is heavier that oxgen therefore stoping the radiant transfer across the glass. Then there the construction of the frame and how many "dead air channels there are. Vinyl windows have a inherainat problem of two things 1) there strengh, partically the sash. Under heavy loads (triple pane glass packs) the sash can become bowed resulting in a bad seal to frame connection, therefore make sure there is somekind of steel reinforcement in the sash frame.
As far as product goes. I of course would want you to buy Republic, but they are or can be expensive. If the house is not worth it (think about resale) then Jeldwin makes some fine products. I agree do not buy Crestline, I firmly belive that Crestline is only around to provide windows for 3 season porchs, and to home builders to cut there bottom line. Bty Jeldwin and Crestline are owned by the same company, I think Jeldwin owns Crestline but I could be wrong.
If you need further assistance please let me know, whatever you buy will be a pretty substancial investment and if you go really cheap, you'll pay for it untill you sell the home.
On the flip side you should get around 83% of your investment in return if you were to sell. Replacing windows is on the top of the return ladder when looking at selling in the near future. And if your not looking at selling soon then by all means, buy something good.