Ok so you have a total of 360w that can be used total combined on both of your 12v rails... but each rail can output 216w (12v x 18amps) max. One rail will be used for your CPU while the other will go to your GPU. So you have 216w that can be used for your GPU (Your CPU should never use more than 144w so your GPU will be able to use its full amount). That being said... You could get away with using even a GTX 680... although that will push the limits of your rail so i would not recommend it. For a better balance with your CPU and other system parts, I wouldn't recommend anything over a GTX 660 or AMD 7870.