Firstly are the timber or are they upvc windows and what type of double glazing do they have at the minute? Do you know the thickness of the double glazed unit i.e 16mm or the more modern 24mm?
I am thinking of upgrading the double glazing in my hardwood windows which are 10 yrs old as well, the glazing in mine at present is 4/8/4 (16mm) "K" glass with standard float to outside and aluminium spacer bar, (I know most companies are using this as standard "low-e" now). When I was ordering the glass 10yrs ago everyone thought that I was crazy as most people had'nt even heard of "K" glass. It would be classed as C rated units now. I am just very interested in conserving energy/heat where ever possible. We have just upgraded the insulation in our attic.
I have alot of glazing in my house and all the glass is toughened as most windows are over 7' high. I would love to have A rated units which comprise of an outer pane with Optiwhite and an inner pane with an Optishield or simular (both panes would have a soft coat) and a warmedge spacer bar and argon filled.
Triple glazing is very very expensive as the three panes have to be toughened (even where toughened glass is not needed to meet regualtions) due to the build up of heat inside the pane. It does not give any significantly better uvalue rating that the double that I described above in A rated. A Triple glazed unit would need to be 44mm thich to get any significant difference in uvalues. It is more for sound.
The A rated unit that I gave above on the Pilkington Spectrum Web Site gives a uvalue of 1.2 in a 24mm unit and to achieve 0.9 for a triple glazed unit it needs to be a 44mm unit which is not typically possible in a domestic situation.
If sound is a problem, use two different thicknesses of glass in your double glazed unit and this will help to reduce sound as the frequency has to change its speed to travel through the two different thicknesses of glass thus losing more of its noise in the process.