Stevec67 is spot-on.
And yes, with a double-glazed IGU, for better acoustic performance a laminated inner leaf is worth it - small uplift in cost for the glass is the best bang for the buck by far. Any laminated glass will improve matters* but specialist types (Pilkington Optiphon etc) have worked on this to maximise the benefit - basically, a thicker interlayer.
If you want to get better than this, fit secondary glazing with a laminated inner (I'd suggest 8.8mm) and some absorption in the cavity between frames as blossomchris suggested. Do this with 200mm between primary and secondary and a melamine foam liner and it will challenge what a brick masonry wall provides, at least - get to a solid point of dimished returns (drop me a pm if you want a detail/spec for it). Recently finished a building where a key consideration is noise from helicopter operations and with a little attention to detail it has proved subjectively really effective (despite noise survey graphs that started at 100dB at 30Hz at 50m ..!)
* two reasons: the inherent damping in the interlayer, which provides a small amount of resistive absorption broadband and more seriously, it kills the 'coincidence effect'; and because the two panes end-up different thicknesses it considerably reduces transfer between the leaves via the gas spring between them.
ETA: Triple glazing units, curiously, is not so effective because it will be 3 x 4mm glass layers and because they are the same size and thickness, with reduced cavities between to meet standard rebates, there will be a chunk of the midband where acoustic isolation is markedly reduced owing to resonant and coincidence effects. Triples certainly can be made to work but still require a fancier glass spec., and as such - end up expensive (because triple-glazing IGU suppliers in the UK are not common - not least because these days good standard IGUs easily challenge a triples' thermal capability). IME the same spent on a double -glazed IGU + even simple secondary glazing will likely romp ahead in acoustic terms. And can be done piecemeal - add the secondary if you think you still need it after living with the base replacement window.
