You do know that you can fit more than one line of text into one post, right?
As for your math? You're right except that the number you get isn't per square inch - it's total. 1440x900 is a lower resolution than 1280x1024, and the same goes for 1366x768. Higher resolution does tax your GPU, but it has to be an actual higher resolution, such as 1680x1050 or 1920x1200.
Meaning the increment must be significant. If it is just a matter of 200K pixels, you may not notice significant increase/decrease in fps.
But you are correct, the less pixels, the less taxing it is to the GPU given the same graphics setting ( Anti-aliasing for example).