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.
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.