Depending on the cpu inside the OptiPlex, for 1080p, I would go 1050ti.
maybe a 950 or 750, unless the cpu is strong enough to not bottleneck them.
search "the 150$ pc returns" on youtube and look at the "linus tech tips" video as it explains this exact thing. (dell OptiPlex with higher end gpu)
if you are able to send me your cpu (the name, not the actual cpu unless you want to and thatll be just fine by me haha) I can tell you what actually performance gains youll get from what gpu.