Double-glazed windows cut heat loss through the glass by around 50 percent compared to single panes, which is why they pay off in cold climates.
Seal the drafts around your current windows first with weatherstripping, since that is the cheapest and fastest win.
Add heavy curtains or thermal blinds as a low-cost second layer that traps a pocket of air against the glass.
When replacing, look for the energy rating rather than the brand, because the gap and coating matter more than the name.
A warmer, quieter home is a pleasant bonus on top of the lower energy bill.