Do directx geometry shaders offer anything over the opengl ones?
What are the oldest cards which support opengl 3.2 and the geometry shaders on there?
What are the oldest cards which support opengl 2.0 and the geometry shader plugin?
Geometry shaders are geometry shaders, regardless of the API you use to access them. They are supported on the same class of hardware: DX10-class hardware.
What advantages does the official geometry shader implementation on opengl 3.2 have over the plug-in for 2.0? Is the official one faster?
The core 3.2 version is not "faster". It's simply a more intuitive API.
For example, core 3.2 it doesn't use the tortured
varying in and
varying out syntax. It also requires you to specify the input and output primitive types directly in the shader, rather than forcing you to use pre-link program parameters. Also, you can use interface blocks to group inputs and outputs together, which makes talking about them much easier.
The only downside really is that you can't use it with removed primitive types (