You overclocked in the wrong tab. If you put the card up from 300/600 that means you changed the 2D settings of the card.The 3D settings are at 400/800. There is a 2D/3D selection tab where the clock settings are.
Other than that, I managed to take my Point of View card to 500/950, I get artifacts in certain games if I use anything higher. Also, try running the Dawn Ultra demo with 8XAA/8XAF, that usually shows if your ram can take the clock speed you set it on.
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