To overclock today we used Rivatuner. All fan settings were left to automatic and stability was tested over a two hour period for each card.
The HD6990 overclocks to the same levels that we have experienced before, which is no surprise as this is the same reference solution. The GTX590 has a surprising level of headroom available, giving an 22% OC on the core and a 15.7% oc on the memory.
Just be sure to use the correct driver when trying to overclock the GTX590. Read more about it over here.