First thing, careful with overheating with the Apple OEM version of the GeForce 2MX. Personally I underclocked it before sticking it in the Cube.
The black borders are probably because the output was a 3x4 screen ratio and the monitor is wide. It may blank out if the system chose a resolution or refresh rate that is not supported by the display.
I don't know what could cause the performance issues? Does scrolling work properly with the Rage Pro?
Panther does have...