Corrigan, replacing windows is a lot of hype unless the glass is falling out due to rot. The payback period is longer than you're likely to live for decent windows, and cheap new windows are generally worse than any window more than 25 years old.

The windows on my house were installed in 1822, and with good caulking, old fashioned storm windows, and a bit of DAP Peel'n'Seal on the sashes, they are more airtight and energy efficient than modern windows. If you have access to a laser or infrared thermometer it will show you where your problems are. I'm a licensed builder; I've been to renovation/restoration conferences; I've talked to window manufacturers. Put your money into your engine after insulating and caulking your existing windows.