Hi, I was wondering that kind of question as well as I am using Dell Optiplex 755 SFF. So last month, I did a lot of research and decided to go for Sapphire HD 6670 GDDR5 Low Profile due to the strictly limit of space and my PSU is ony 275W. It works fine for me. I am an occasional gamer, the graphic card works great for games like Portal 2, Skyrim, ... and multi monitor is also not a problem.
However, if you are not space limited, I suggest you go for ati 7750.