Unless it is a particularly lousy 750W, a 280X should be no-sweat - the 280X's real power requirement is under 200W and the rest of a typical system usually needs well under 150W too.
On a good 750W PSU, you could even have a pair of 280X.
Usually 300W is left for everything except the graphics cards - cpu, ram, mobo, fans, hdds, sdd, water cooling. odd, usb - printer, mouse, keybpoard, speakers, headphones.
The add 2 x tdp (for 2 x 280x's = 2 x 250W = 500W) gives 800W at least