relies heavily on the card's cooling and design. some cards can be pushed and over-volted a lot more than others, and some may last only a day. typically, the core can take more voltage than the onboard memory, but it's still a good idea to have adequate cooling; i wouldn't touch voltages at all. there's no real practical reason to do so, and usually it's just a matter of time before they quit, like kitty said.
which video card is in question, or is this just a general inquiry?