I am running a stress test on mine at 900mhz core clock.
good to point out though that my gpu got this fancy heatsink called WindForce 3x which looks just like this:http://img.hexus.net/v2/graphics_cards/nvidia/gigabyte/GTX570SO/CardB.jpg
which is very effective, tests showing it being able to sink the temp up to 20º in compared to stock (MSI, EVGA) gtx 570.
got it stabilized at 74ºC with Kombustor after 20min, but doing some intuitive geometric extrapolation i say that it could reach like 77ºC (+-0.5ºC) after some hours running it. keep in mind that Kombustor puts UNREALISTIC load at the GPU, so playing demanding games, gpu load might be significantly lower.