Best bet is to visit a GPU list like this: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
Find a good deal on the most you can afford. Avoid going lower than a 7770 or R7 260X for Radeons or anything below GTX 750 for nVidia. Go for 2 GB of GDDR5 memory at least. Look for deals, be patient, and jump on what looks good to you.
Make sure the card will fit into an optiplex case, it may have to be the slim version. Otherwise, don't use the optiplex for gaming.