quality windows?

[Selina Rifkin]

We have purchased a rehab property that we will live in while fixing up and then rent out. We plan to hold for about 7 years. The house needs windows badly and a salesman came and demonstrated why expensive windows are expensive (heat loss, construction etc.) They are a 40 year old reputable company that gives a lifetime gaurantee on their product. This stays with the house and they will replace even if a window gets broken.  But of course it is expensive. My question is how much do good windows increase the value of a house?