Userbenchmark.com optiplex 380 https://www.userbenchmark.com/System/Dell-OptiPlex-380/1942
The 710 won't be a whole lot better than the onboard GPU. You didn't say what size opti 380 you have. But I agree that the 750Ti is a good minimum card.
With 4GB RAM get a 1GB GPU, if you upgrade to 8GB then a 2GB GPU is good.
The top one on the list is mine. X5470 3.33GHz, 8GB DDR3 1066, and GTX160 3GB.