Right click on your desktop, click on Catalyst Control Centre. Click on the 'Graphics' tab in the top left corner, Select 'ATI Overdrive' Click on the padlock and then you can adjust the Core and memory speed.
If you want to mess around with voltage then you need a program called MSI afterburner.
Every HD5850 overclocks differently so I can't say to you "Use 1.2V and 900Core and 1100Mem."
The best way to go about it is to increase the Core speed by 10Mhz and test for stability with Furmark. If its stable after 15Mins then increase the speed by another 10Mhz and test again, do this until it becomes unstable or you start to see artifacts, at which point you can either:
A): increase the voltage to regain stability
B): reduce the speed by 10-20Mhz to regain stability and call it a day.
Remember to keep an eye on your temps with GPU-Z, make sure none of the temps exceed 90C