It is essential the graphics card is supplied with its specified voltage. This is normally determined by the type of AGP slot the card fits into, but most recent 4x and 8x cards will be 1.5v.
If one of the newer generation cards requires more power, it is supplied with its own power connector, that is plugged into one of the power supplies 12v connectors, and is then regulated by circuitry on the board, as required by the designers.
On NO ACCOUNT over-volt a graphics card. It's an expensive lesson you wont forget.