Quote from your graphics card page, found here.
Minimum of a 400 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 26 Amps.)
Quote from your PSU page, found here.
As you can see, your power supply is under the minimum recommendation for that graphics card. Your card needs 26A and your PSU is putting out 24A. Don't forget this is the minimum, you should have a bit more headroom just to be on the safe side anyway.