Your 6800GT should get much of its power from the molex connector, how much are you overclocking? I haven't found it necessary to increase my voltage to overclock my card. However, with older cards this was necessary.
Right now im running 4.16 / 1.17 and its very stable, But whenever I put either one any higher it says test failed (the built in mandatory nvidia test). Im assuming maybe I need to crank up the voltage a little?