In general there are no specific limits, as you say it depends on the computer how well it will cope.
The only two limits that I know about, is 65,536 points in one line. (but can be split into many smaller lines)
And the texture size for groundoverlays. Generally graphics cards have a specific size limit on textures. So ground overlays bigger than the texture size will have to be resized for display.
(can find your texture size using the standalone version of Google Earth, in the Help > About dialog)