Standard 6800 Ultras run at 400/1.1 and my pair of XFX's go there easily. Add some additional cooling and I have seen guys get them up near 490 but I haven't heard of one at 500 yet. My 6800GT's run really well and relatively coolly at 400/1.1 and I don't push them past that as I haven't had a need to (can play everything maxed out) and want them to last a couple years.