If the fuzz felt on your slider window has worn out to the point where the windows rattle in their channels... a couple years ago I got some replacement fuzz felt from HD and retrofitted most of the sliders here. It was a major improvement - no more rattling in the wind and I'd guess there's less draft. These are all aluminum frame single horizontal sliders - at least 40 years old, I figure. Prior to that, the place probably had steel frame casement windows, which I actually would prefer but those have their own problems.
The tricky part is pulling out the old fuzz felt. It requires full dissassembly of the frame and glass removal. I most cases I was able to do this and save the glass, but a couple of windows were stubborn and I had to get new glass. Again not a biggie, since these are single glaze design.
If you look at energy audit info, windows are actually a rather small portion of the building envelope. If the windows are not leaking air (which a lot of old windows can do), then the actual energy loss through the glass, even single glazed, is relatively small. And it can be minimized with proper shades and curtains, or if necessary, storm windows. This SF Bay Area climate is mild enough that personally i don't think double glaze is really necessary. But if one is going to replace windows anyway I suppose it won't hurt.
The biggest energy drain in an old house can be air leaks - through windows, doors, exhaust fans, electrical outlets, etc. Also chimneys. Next up is attic insulation. This '41 home had zero attic insulation when I bought it, and lots of air leaks in all the above categories. In the winter, the 100,000 BTU forced air furnace would run continuously, and it was still uncomfortable. Methodically I sealed all the air leaks I could find (some were gaping), and then put in a total of about R40 in the attic. Later I added R13 under most of the floor (wood over crawl). After all that the gas consumption in the winter dropped to about 30% of what it was before, adjusting for average monthly temps. And that's with leaky aluminum horizontal single pane windows!