Mip-map dithering's only relevant when you *can't* use trilinear in a game. It's an approximation of trilinear left over from previous Voodoos where actual trilinear wasn't fully on the table in most cases. It also doesn't get along with multitexturing well so you'll want to disable it.
To enable trilinear, have this in a config or type it in the console:
gl_texturemode GL_LINEAR_MIPMAP_LINEAR

Thanks. Is there a reason I should type it in the console if the driver utility allows me to select it (among other options)? And what sort of game really cannot user trilinear?