Try PyADL. It can be a bit trick to get working but so far I have successfully used it to set core and fan speed and it says it changing the voltage.
http://forum.bitcoin.org/index.php?topic=8451.0
Topic: Increasing GPU voltage in Ubuntu (Read 2890 times)
Use the latest CCC. It should let you go beyond the limits 
someone made a ati overvolt utility in linux on the forums here just search for it, however it has really limited support like only certain voltage regulators on the 5800 series work with it. Other than that you have to flash the bios of the card to up the voltage.
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Anyone?
bump.. So is it true that it's impossible to overvolt GPU over the stocks limits in Ubuntu?
How can I increase GPU voltage in Ubuntu?I know AMDOverclockCtrl can do it but it can't go above the stock limits. My GPU is HD6850. I tried to modify the GPU bios with RBE (on Windows) and even the slightest change caused the speed drop from 250Mhashes/s to 25Mhashes/s, no matter what I did, even if I just changed core clocks. Also AMDOverclockCtrl didn't regonize core/memory clocks or voltage anymore and Aticonfig overclock stopped working. Then I flashed back the unedited/original bios and everything worked like before. Any tips?
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