gtx 750 t.i. or r7 260x. The gtx 750 t.i. is the lower power usage of the two,the 260x uses a little more power but is faster.Oops, no, it just uses more power.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
You could go a bit higher power wise, but I would recommend a bigger power supply for long term. 260x,265 and 270 power figures here;
http://www.bit-tech.net/hardware/graphics/2014/05/08/amd-radeon-r7-265-review/1