Moisture, whether droplets or "fog" inside the windows does indicate a failed seal, and the only repair for that is a new sealed pane unit. Depending on the state of the frame that can be only the sealed pane unit (same as you'd do for a broken pane) or the whole window.
It is normally less cost to just replace the sealed pane unit than the whole window, and that will (depending on the replacement unit) get you the bulk of any energy improvement you might get (there are, depending what the old frames do, possible improvements in thermal breaks on the frame itself - or those may already be in place in your old frames) - if you don't notice the frames themselves being particularly cold, or having a lot of condensate on them, they probably do incorporate a thermal break.
If there are major mechanical issues with the old frame, or the flashing is leaking (which often leads to problems with the frame) then replacing the whole window might make more sense than just replacing the sealed glass unit.
If you are trying to evaluate "how to save money on energy" and where replacing windows or panes would fit in that, many places have free or low cost energy audit programs sponsored by the state or a utility company, and in most cases it's true that replacing the windows (unless visually offensive or disfunctional - i.e. not for energy reasons as such) is far down the list, with boring old "add more insulation and seal air leaks" being right at the top, unless you've already done that.