Optiplex 755 DT requires low profile card or card riser, which is very difficult to find, so your choice http://www.newegg.com/Product/Product.aspx?Item=N82E16814202052 is this correct
, would not fit, power wise it would work.
This card a bit more expensive, MSI N750ti-2GD5TLP GeForce GTX 750 Ti 2GB 128-Bit GDDR5, but stronger and will fit and work in your PC with an existing power supply, which is 280 watt
Anything else I can help you with?
P.S. If you find any particular answer very helpful, please select "Pick as the Solution
" green button, so other users can see it as a reference. This will make this thread SOLVED.