Your video card should have a polygons/second rating...
That being said, DarkGDK seems pretty fast compared to C# Direct3D...but how you write the program matters a lot. If it's worth it, swap models when things are further away. (From a hi-poly model to a low-poly model based on distance from the camera...) There may or may not be a built in feature in video cards that does that exact thing...also I think there *IS* a way to render the skybox without having a large draw-distance, but I don't know how to do it in DarkGDK! In DirectX, you simply draw the skybox before you draw anything else.