MSI 260GTX Afterburner will not allow core clock over 704.
I'm trying to overclock my 260GTX, but when I try to apply any core clock above 704 it will not allow it. 704 is as high as it will go, and at 704MHz I get no artifacts running furmark at the standard benchmark settings.
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The card might be keeping you from overclocking it higher because it doesn't think it can run properly at higher settings. I wouldn't suggest pushing the limits, especially since GPUs are not as overclockable as CPUs.