The only limitation is cpu after you remove chassis physical limitations and psu restraints.  For i7-4770/4790 some of the good gpu matching include RX 580 and GTX 1070. “Best” depends on settings and resolution you play at.  You can use gpu check to get an idea of cpu bottleneck and fps at different settings and resolution. [URL] [URL] GTX 1070 good for 1440p resolution. For 4k resolution you need a faster gpu.  
What is the best graphics card the motherboard for an Optiplex 9020 SFF can handle? I'm moving everything into a bigger case and replacing the power supply with a better one, are there any other limitations on the usable graphics cards.
Optiplex 9020 SFF motherboard, best compatible graphics card