I actually have that same psu or at least a similar one. I just got a 280x, and after about a week of it working it stopped delivering enough power for the gpu then outright died. So I would say it's a bad idea from experience. Maybe if you are running no overclock like I was it might stay under but I doubt it. My overclock was not using that much extra power on my i5 3570k. So after a bit it failed from stress and being 2 years old and on all the time. With all the stuff running on the 12v rail you are not left with much power. Check your amps if they are high enough you should be ok. 280x requires 30 amps, so make sure you have 30 amps available for the card alone.