this is the highest stable I can get with ati tool. gtx 260 bfg tech vanilla version. I have it flashed by editing the bios. I did check the clocks first, even 1mhz higher on shader clock after 1533 gives artifacts so its seems that there might be some kind of hard lock, the core clock can go higher, I was able to unlink the clocks and get the clock as high as 771 but for some reason it did not let me do that again, I think you have to maintain the shader/coreclock ratio, I just cant get the clocks any higher than 750 without raising the shader clock, so the shader clock is limiting the core clock, here is a screenshot.
the fan is set to auto, and it goes high up according to temp.