You should use Display port or DVI-D for best picture. HDMI works but you may want to read technical specifications on the monitor to ensure that it accepts HDMI at 2560x1440. (The dell U2711's do not).
The cable DVI-D cable should be the same on both ends, mine specifically is stamped "duel".
As for power you will be in good shape. I run 3 dell U2711's on a GTX titan, so one 2560x1440 on a 780 gtx should be fine.