Generally modern gfx cards do transform and lighting in hardware. Thus they have the vertex/index buffers (the geometry) and textures all loaded up into the gfx cards vram at start. Thus the bus only has to cope with instructions on what to do with the geometry, not the actual verts as you would if the CPU did the transform and lighting.
Of course if you start doing dynamic geometry textures on the CPU this changes the situation. Although with vertex/pixel shaders you can do a lot on the GPU.
