I flashed my 3 5830's to 975/300. They all run around 280-285. I know I could oc more but I was having temp issues at first so didnt' want to push it too high right now.
What I did :
Download Radeon Bios Editor http://www.techpowerup.com/downloads/1962/TechPowerUp_Radeon_Bios_Editor_v1.28.html
Download GPUZ ( to save bios file ) http://www.techpowerup.com/downloads/SysInfo/GPU-Z/
Download ATIWinflash There's a link in RBE that says " get winflash ".
First, Open GPU Z and there is going to be a save button to save your bios image. Save it. Then open Radeon Bios Editor and click load bios. Find your previously saved bios. There are alot of options you can do. I just did this really quickly and only changed 'clock 0' in the overclocking tab to 975/300 and then save bios. It will save the bios. Then open ATIWinflash, click load bios. IT should display all active cards. Check the one you want to flash and click 'program'. This step only took about 10-15 seconds per card and then after reboot they were all OC'd.
Full Radeon Bios Editor tutorial here : http://www.techpowerup.com/articles/overclocking/vidcard/154