I'm sure everyone knows this, but a little reminder.

Increasing the voltage to your GPU or your CPU can have negative consequences. Such consequences may include. . .

-increased power consumption
-instability
-reduction in processor life
-increases in heat output
-voiding your warranty
-stability in benchmarking, but instability in other applications
-stomach pains
-causing damage to your mainboard (rare)

If you're overclocking make sure you keep a close eye on temperatures and if possible use aftermarket cooling methods (typically a better heatsink/fan or combination of both).

If such issues crop up discontinue your voltage increases. If the issues persist after returning to default settings, consult a nerd.