Take a card like the 680 dc2t, 8pin & 6pin plugs which means you only can get 225W (150+75) from the cables but to get more power you'll need it from the pci express slot. At 160 power target or either 130% TDP the card pushes 350W from the wall, so you're basically heavily pulling power off the slot so I don't see the problem at all. Cards tdp values
http://www.techpowerup.com/vgabios/1...48-120709.html
Most OEM psu's have around 250W and the tdp difference is 2Amps only between the 7750 & 7770 so it doesn't make sense that his psu wouldn't take a 7770. His system should take
150W in Cinebench 11.5 but in normal scenario's it would never get that extreme so lets say it's around ~100w and an additional 80W shouldn't be a problem at all.