With a graphics card its not so much about the total wattage of the PSU, The main thing you need to know is how many Amps you have available on your 12Volt rail as this is where the power for the card will be coming from. If your card wants 28 Amps and your PSU has 28 then you will be fine it dosent matter what the wattage of the unit is.
But basically there are about 4 main ways it could go if you try to run a card on a PSU that hasnt got the power to run it.
1 The card could run but at a reduced power/efficiancy so you dont get the full performance from the card, some cards will do this its called throttling back and they do it to avoid the damage that can be caused to the card by running it underpowered.
2 The card may just not run at all
3 Wont be a problem the card will run fine, manufacturers are usually very cautious and over quote the needed power to cover themselves
4 And this is the main reason why i say DONT DO IT . The card may go bang big time and take the rest of the computer with it.
Im not trying to be over dramatic or scare you it is a real possability that 4 will happen if you are not sure the psu will cope get a better one.
Mactronix