5/8" is 2x as dense as 1/2" wallboard.
In stores in my area, the drywall sold as "Fire code" is twice as heavy and dense as the next smaller size, i.e. 1/2". So not only is it thicker, it is also denser per area, or volume. It stops noise better since it is denser than the light stuff.
Above, RRW is suggesting again this same recommendation:
- some dense material
- some air space
(either real air, or foam which is trapped air, or a fluffy rigid panel which is trapped air).
In addition, he is suggesting a good way to de-couple the studs from the wall panels.
To go a bit further, you could also use FOUR parallel 3"x2"s instead of two 2"x6" as ceiling and floor plates. Hope that is obvious why to do that. I know an experienced remodeler who did that and he claims the benefit (the gain) was worth it. He is only half way through soundproofing a big apartment building, after a few years, working off and on as apartments become available.
If you use two layers of any (same) material, it is a good idea to use different thicknesses, because a certain part of a wave (sound wave) can find its way through one thickness because that thickness corresponds to a "harmonic" multiple of that wave, and to use the same thickness of the same material will only aggravate the transmission problem instead of damping it. So don'T double the same panel and expect it to work twice as well for all frequencies; some parts of some voices or other noises will travel through better than anything else, and it will feel real loud if that is all you can hear.
Closed cell foam causes a bit of this "harmonic" multiplier effect, since each cell is tight pressed against the next, whereas open cell foam does not have this characteristic, and thus does not let sounds reverberate through. Soft foam made for door and window sashes is great stuff to block all air leaks around wall openings (e.g. for electrical boxes and switches) for soundproofing since it is open cell foam (Expanded, not Extruded). (EPS not XPS).
David