6970 LOW FPS?

[GraphicsKid]

Guys, But what SSAO truly do? I tried with medium - 16 and High - 64 there isn't so much difference
Just a lot of lag.

Try this: grab a box and hold it in front of you. You see how the box seems to have a fluffy shadow around the surrounding edges (even if it's nowhere near a wall?) That's the Screen Space Ambient Occlusion in action. Those numbers -- 16, 64, etc. -- are how many samples the shader takes to calculate the occlusion effect. The fewer samples, the grainier the shadow looks. Once you get to around 16, there's not much difference in visual quality.