I'm using 2 GTX 260 Core 216s. I ran 3d mark vantage with them overclocked to a core clock of 711 and a shader of 1528 and got a frame rate of 18.56fps. Then I tried at a higher rate of 730 (the shader is connected to the core) and I got 6.87fps. I tried 720, 715 and 711 and got all exactly 6.87fps. What's wrong? It probably wasn't stable at 730 but there wasn't a crash or blue screen.
i think it was'nt stable because an overclocked video card and is not stable will turn out slower rather than faster.i think it affect on what processor you use as well.if you are using a not to good processor an overslock is useless you won't see any difference or else yours rig a overheating which makes it slow aswell