For overclocking today we used the latest beta version of MSI Afterburner 2.2 which allowed us to increase the voltages.
By increasing the core voltage by +87 we achieved a stable overclock at 863mhz. It was possible to improve this a little more, but we noticed some very minor artifacting which could lead to long term problems. We aren't chasing impressive figures on this page, but realistic 24/7 speeds. Due to the voltage increases core temperatures rose by around 3c under load.
The overclocked speeds translate in a rather healthy performance boost, rising from 6,250 points to 6,844 points in 3DMark 11.