It depends, I assume the graphics card is designed to be overclocked and if so then it might well work perfectly at the settings you propose. Of course the big danger is overheating, if you overclock it will get hotter quicker.
You would just have to try it and see what happens.
It depends on the heatsink and fan on the graphics card being built to take the extra heat and the amount of cooling the case can give IE can it get rid of the hot air quick enough to stop the components from blowing.
And personally if the graphics card is not designed to be overclocked I would not even attempt it - unless you can afford to buy a new one if it goes pop.