Graphics Card for a 755 Optiplex DT

Solution
You cannot fit those. Dt requires low profile. You also are limited to a single slot, short double slot cooler. Those gpus also won't work with your psu. Why are you upgrading? The only things better than a 550ti that would work with your case and psu are 750, 750ti, 950 75 watt, 1050 or 1050ti.
Go with an MSI low profile GTX 1050 ti, it will fit in a lot of small form factor cases and works better than what you listed, RGIHD (RandomGamingInHD) put out a video on the 1050 base low profile one, and it's perfect.
 
You cannot fit those. Dt requires low profile. You also are limited to a single slot, short double slot cooler. Those gpus also won't work with your psu. Why are you upgrading? The only things better than a 550ti that would work with your case and psu are 750, 750ti, 950 75 watt, 1050 or 1050ti.
 
Solution