If you have older wood windows, the weight pockets are not insulated and should have a polyurethane foam injected into them.

Any window with Low-E will be sufficient,however, you should continue to do your research and find the best window for your application.

Your air leakage issue should be addressed with a good replacement window. Some are much better than others when it comes to air leakage.

There are no weight pockets (the house was built in 1968). But it looks like the builder failed to insulate in the cavities between the window frame and the next wall stud.

Does that mean I need full frame window replacement if I want to get those spaces insulated?

Or is there a way to get it done with just insert window insert?

What is the price difference typically between insert replacements and full-frame replacements (assuming it's from the same company). I realize there are a lot of variables, but assume it's the same style window from the same manufacturer.

Doing a full frame replacement will still not address the void space because you are not removing structural members during a full frame replacement.

What is the exterior of the home (i.e. brick, siding, stucco, etc)?

You can certainly insulate that area with an insert window application and some injected foam. Depending on the exterior trim, you can likely remove it and inject foam or insert fiberglass. Foam will work better as and insulator and air barrier.

It's mostly wood siding, but our most problematic window in our living room has brick on the exterior.

So the only way is to inject foam insulation through the walls?

As a contractor, here is what I would do.
Rip out your sheet rock from the inside of the house. Insulate the space with R14 fiberglass insulation. Caulk any exterior cracks you see. Get a custome interior window (Crestline, Modern, Jen-Wels, etc) and install it from the inside. Caulk insde and out. It won't leak.

I have used foam insulation and I don't recommend. In one case, we had a house foamed with "Rapco" foam and it blew in the wall as it expanded.

Also, it actually shrank in size (years later) and left a 1/2 to 1 incg gap around the area that was foamed.

The expanding foam can bow a window frame(or a door frame). Not a good choice.

I'll make an assumption that the void area is the small space between the window frame and the framing stud to which the window anchors. If so, this space is exposed by either removing the interior wood casing or exterior wood casing. On the interior, you may also have to cut about 1/2"-1" of the edge of the sheetrock back and away from the interior perimeter of the window frame. Once this small void (1/4"-3/4") is exposed, it is easy to either stuff it with fiberglass insulation or spray foam into the void. If you elect to use foam, it must be done in a controlled manner, meaning to spray about 1" thick at a time, and apply several coats as each completely cures -this will avoid what the poster mentioned above.

To fix the window air-infiltration problem may also be cured once you have this void exposed, and before you insulate the void. Check the window frame to ensure that they are installed square, as an out-of-square window will not seal properly. Using shims, you should be able to square the frame by inserting the shims into the void (i.e. shim space).

If the windows are square, check the weather-stripping to ensure it is in good condition. A second remedy is to consider exterior or interior storm panels. Your last resort would be to replace the windows.

Just to clarify one point. The foam insulation that was recommended is called low expanding foam. This is not the type of foam insulation you purchase at the hardware store that expands to 6 times the amount you apply. Low expanding foam will NOT warp the frames if applied correctly by a professional.

Here is a link that might be useful: Hilti Foam