When you overclock your graphicscard you normally do it by overclocking core clock. But you can also overclock your Memory...
But how much FPS can you achieve with it? See here
Note: The GTX 970 has 3,5GB fast memory and 0,5GB slow memory. In this test I wanted to know how well the performance will improve when a game also use the slow memory too. The next test will be Assassin´s Creed Syndicate under 3,5GB
Performance Boost: 13,3%