It really depends on your needs and budget. Replacement windows will enhance both your home's curb appeal and its energy efficiency, so if you can afford to upgrade all of your existing windows with the latest and greatest, it will benefit you over time (especially in resale value). However, that's not often necessary, as replacing windows room-by-room, floor-by-floor, or just in the front, back, or sides can still make a big difference. If aesthetics are what you're after, just one set of bay or bow windows can totally transform the look of your home.