Forgot to add: OpenGL is the fasted on Nvidia/AMD. I haven't played around much between D3D and OGL on my HD 4600, but that driver issue mentioned above is on OGL. Try both settings, and see what works for you.
You can bump up your IR and AF on OGL; and your IR, AA, and AF on D3D
You can bump up your IR and AF on OGL; and your IR, AA, and AF on D3D