If you want the right graphics card for your system we need to know your budget and the wattage of your PSU and also the make. But in my opinion if you are looking at the range of a $100-120 graphics card a Radeon R7 260X 2GB GDDR5 for $104 would be a great option, If you are looking for a low end card price range from $65-85 go with the R7 250 2GB DDR3 for $83 OR The Nvidia GT 730 1GB GDDR5 for $73
The R7 260X needs a minimum of 500W listed in the AMD website. but 450W I think would be enough.
R7 250 only needs a 400W or less in my opinion
The Nvidia GT 730 can run at 350W even maybe run at 300W
Make sure that the +12v rail has enough amps to support these cards, cause the R7 260X need 30A the R7 250 need 20A and the GT 730 need 22A