No. Stop. Any 750w with only two 6 pins is a cheap power supply. Even my moderate quality 550w has one 8 pin and a 6 pin. What model/make is it? http://www.youtube.com/watch?v=86ss7I3nhoc
You actually do not need an adapter.
Anyway, card won't run at full performance with an adapter because it is still only a 6 pin from the PSU. It isn't going to magicly create the energy difference between the 6 and 8 pin.