You probably care less about actual size and more about pixel count (and aspect ratio.)
The Apple 20" is 1680x1050; 17s are usually 1280x1024.
So by pixel count, the two 17s are some 48% larger than the 20". Note that you will not gain significantly in the vertical dimension; it's just a single screen that is quite a bit narrower than your two screen combination. Whether this is better operationally for you depends on how you like to lay out your screens.
Of course, two 30" displays would be the bee's knees.
As far as screen area, just having the diagonal measurement (or any single linear value) is not enough information to calculate it. A 20" diagonal screen that was only an inch tall wouldn't have very many pixels. This (and the fact that the number seems bigger than it really is) is why they use diagonal measurements, something pioneered by the TV industry many years back.
If you had two numbers, you wouldn't need trig anyhow. Either a straight multiplication to get the area (if you know height and width) or reshaping pythagoras (B = SQRT(C^2 - A^2) ) to figure out the length of the other side knowing one side and the diagonal, then multiply. But this is neither here nor there.