The Optiplex 960 is a tough nut to crack as far as GPU goes. It's a BTX layout so the GPU goes towards the CPU cooler not away from it. To make things worse they added a couple PS2 ports next to the Printer port so single slot only at the back. A Zotac GTX1050 Mini will fit an Opti 760, but those 2 ports on the 960 get in the way.
You didn't say what size case your 960 is. I don't see any way to get a 570HD in there. You might get a single slot Low Profile GTX750 in there with a tall bracket.