Mismatched monitors are best done deliberately, with intent. Barakta's current setup, for example, is a largeish 16:10 monitor with a smaller 4:3 one in portrait mode beside it. They're almost exactly the same height (plusminus bezel thickness), and pixel density which avoids resolution shenanigans, and I reckon it'd be a pretty nice layout if the colours matched and you were doing something videoish (which, sadly, barakta isn't).
For general use, I'm of the opinion that you either want 2-3 matched squareish monitors, or one huge one to give a similar area and resolution (and a window manager that can handle this sensibly, which I'm reasonably sure Windows isn't). Trying to arrange widescreen monitors side by side is just awkward, unless you go portrait for some of them (which is fine for things like code and email, but annoying for more graphical stuff).
Given that 5:4 monitors are deeply unfashionable, I'm holding out until I need computer glasses and can justify something big and expensive.