If you already have a 280X I wouldn't really recommend replacing it with a 760 - unless you have another home for the 280X. The 280X is hands-down the better card, it probably only consumes about 30W more. A 760 will definitely munch more than a 7870 which barely even needs the second 6-pin (as evidenced by the R9 270).
If you're looking at a 250D then you really have nothing to worry about. People run Titans in the SG05, and the ventilation on that is hardly perfect for high end cards.
If you have atleast a couple of intakes, or a couple of exhausts then the air moving through your case should be able to carry out an extra 30-50W with very little effort.
---Engage annoying Physics pedant mode---
Please capitalise 'Watts'. The dude deserves the respect