What's the differance between Google Earth (DirectX mode) and Google Earth (OpenGL mode)?

The only real difference is that opening in "openGL" GE is using the memory that is dedicated on your graphics card, not your system resources. If you have a graphics card installed in your computer that is capable of handling the memory requirements of the GE (Google Earth) program, you open with "openGL". If you computers graphic card has to use you systems memory (RAM) then you would open using DirectX. This is Microsoft way of letting you enjoy the 3D effects of many programs. When you first install and run GE for the first time, it should tell you which mode to open in and then set it as your default. If you are still unsure, you can always try to open in both modes to see which gives you the better performance...