Actually, the 20" silver part of the computer is .94" (2.39cm); the 24" is 1.24" (3.15cm).
That information is found on the apple store imac gallery. I would guess the black would be a little more for each. (maybe the 20" is 2" at the hinge, and the 24" is 2.5"?)
-=|Mgkwho