The New Maxwell based GTX750 and even better the 750 ti uses 60watts and under , so it does not need a PCI-E power connection. They cam be a bit pricey but when it comes to performance per watt as of now they are the best cards on the planet that use only the power from the PCI-e 16x slot.
Up until the GTX 750&750ti came out I would always recommend a AMD hd7750 with GDDR5 memory. I have a HIS iCOOLER HD7750 w/1gb GDDR5 and I can play almost anything in 1080p with medium settings and get 45-65fps in crysis 3 and BF3. For 80-90$ the HD7750 uses around 48 watts of power. Mine overclocked like a beast from 800 core to 1150 and 1125 mem to 1350 makinf it perform as well as a hd 7770 at stock settings.
So I recommend If you have the money for it get a GTX750ti or the GTX750.....EVGA makes the best NVIDIA cards and has the best customer support . If you so happen to have a good PSU with a 6 pin power connector for a GPU then the EVGA GTX 750TI FTW 3GB card has the power connector so it can be overclocked much higher then without a 6 pin connector. I have seen core clockspeeds in excess of 1400mhz on the core adding a good 20% in performance!!!
If money is a issue the the HIS Radeon HD7750 iCooler , 1GB GDDR5 will do you well along with the HIS Radeon R7 250 iCooler Boost clock1GB GDDR5, with the 250 having only 384 shaders to the 7750s 512 shaders . If you have a PCIE graphics card power connector NEWEGG has this GIGABYTE GV-R726XOC-1GD Radeon R7 260X 1GB 128-bit GDDR5 card for $120 that comes pre overclocked to a impressive 1188MHz core and 6500MHz memory and is twice the card the HD7750.
Ill keep my SLI'ed EVGA GTX 770 Classified 4GB ACX cooled cards. They are works of art and I had to replace my 2 EVGA supeclocked
GTX 560 TI cards in my 3.3 year old 24/7 4712mhz i7-2600k p67 rig with the CPU running at 1.355volts...5ghz needs over 1.42 volts and my 120mm radiator does not keep it cool enough for me to be comfortable running it a 5+ghx 24/7....4712mhz 46x102.6 I think it has been since the classified cards were released since I messed with it.
Its kinda nice knowing that a 3.5 year old SANDY BRIDGE i7-2600K CPU running at 4.5 ghz is within 2.7% the gaming performance of a IVY BRIDGE I7-3770K CPU @ 4.5ghz and a HASWELL i7-4770K CPU.
High Index
Intel Core i7-4770K 4.5G 102.70 %
Intel Core i7-3770K 4.5G 102.74 %
Intel Core i7-2600K 4.5G 101.68 %
Intel Core i7-4770K Stock 101.57 %
Intel Core i7-3770K Stock 101.98 %
Intel Core i7-2600K Stock 100.00 %
More is better
Intel Core i7-4770K vs 3770K vs 2600K Gaming-Performance review. Great article I am posting a link if you want to check it out.
http://www.ocaholic.ch/modules/smartsection/item.php?itemid=1164&page=13