Its not the temperature which it can withstand - it is the internal psu temperature at which measurements are done. So if a PSU is tested in a real cold environment, you'd be able to pull more power out of it than a hot environment.
Antec's low end stuff is normally rated at 40 degrees. 400W is enough for a 960 to be honest. You don't neet 500-550W. I'm running a 7970 GHz edition card on a 550W and it doesn't break a sweat.
So should i go for Antec vp550p ?