replacing double glazed units

I want to replace a couple of my windows which are misted up. So in order to avoid appearing a total amateur[1] I need to know -
How to order the units, ie are they specified as total thickness of the unit or as the separate thicknesses? ISTR hearing that some windows have a 6mm and a 4mm, and some need two X 6mm. Also that some have to have toughened glass.
And how many kinds of the rubber seals are there, will I need to provide a sample?
Any links to good DIY sites or suppliers would be appreciated.
TIA
--
Steve

[1] Which I am, but I don't like to advertise the fact.
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If it's just the glass units need replacing it's best to find a local firm that actually makes them up. There aren't that many but you may avoid pointlessly paying a markup. They are normally specified by three thicknesses, glass, gap, glass so referred to as 4-6-4 or 6-10-6 or something. There isn't normally any reason to have different thicknesses of glass inside and out but a lot of units have coatings on the glass these days so need to be installed the right way round.
If you need gaskets hinges or locks you could try here:
http://www.hdhardware.co.uk /
Tim W
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http://www.steamywindows.net /
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