The NVIDIA GTX 750ti FOR SURE. It consumes little power and is a beast on its own (ok, not a beast, but still pretty good). It is also a bit more efficient than most graphics cards. A lot of people may argue for an AMD r9 270 which is also really good and actually outperforms the 750ti in SOME CASES. Like Shadowblade2652 mentioned, it is the resolution that counts. May I ask if you are gaming?
A 270 will work just fine in a lower price (A 270x is there but its the same thing with just a higher frequency-get a 270x if you are not so keen on GPU overclocking). I'd recommend the 750ti but if possible, but aim to spend a bit more for a 280. It comes with 3 GB of VRAM instead and performs great (Once again, credits to Shadowblade2652).
I'd say get the 750ti. IDK, I just like it so much...