In most situations, replacement windows are NOT cost effective from an energy standpoint. From analysis of several houses using energy auditing software, I see realistic savings closer to $50-$100 per year. In one recent house with a replacement window estimate of well over $10,000, the savings were estimated at $52 per year. You’d be better off putting the money in the bank and drawing 1% interest.
I do see a couple reasons for considering replacement windows: 1) You can’t see through the old ones, 2) The old ones won’t open, 3) The old ones are rotten, falling apart, or expensive to maintain, 4) Personal belongings are subject to fading, or 5) You have a lot of unshaded windows facing east, south or west.
Be aware of a couple items. The glass in replacement windows will probably be smaller than in the old windows. The light coming in may be less, especially with Low-E coatings. Air infiltration may change significantly, which may cause some borderline situation to become problems.