um yeah for 5850 you really cant go to far past 800 with out a slight voltage jump
Its reasonably safe (assuming you run the fan faster) to increase the voltage in between 1.2-1.3v (once again assuming you run the fan faster to keep VRMS and Core cool) and that should easily allow you to hit the 1 ghz mark.
Granted not all 5850s are the same so you might get more or less but it seems that most people can hit 1 ghz with 1.2-1.25vs
Most people also say that increasing the volts past 1.3 isn't really helping and that around 1 ghz is the max stable OC
Personally I'm using MSI afterburner with the core at 1 ghz, with 1.25vs, with a custom fan profile that has the fan idling at 35% and going to 100% at 80 to keep everything cool