Oh boy.
Assuming the aperture and shutter speed are constant (we'll say 1/15 @ f/2.8); doubling the ISO (ISO 200 -> 400) would yield a brighter picture. By doubling the ISO, twice as much light is being collected. Full ISO stops are 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600. One stop is equivalent to a doubling of light. ISO 100 -> 200 = 1 stop/double the light; ISO 200 -> 800 = 2 stops/quadruple the light.