if you're having voltage problems, it's usually from VRM. can you link the exact model of your card please?
I have a pair of 460s and i can usually do 800/1900 at 0.987 and 900/2000 at 1.087 (although this isn't 100 stable due to summer, it should be more stable with winter temperature)
also, besure to raise the voltage slowly and carefully. if you give it excess voltage the GPU may die very quickly; although the upper dangerous ranges are usually BIOS-locked by default.