Upgrade Optiplex 390 for gaming

Hello everyone. I'm using Optiplex 390 MT with i3 2120 processor. I want to play game using this pc. So right now I just upgrade the GPU to GTX 1050 Ti only. The RAM is slot1 4GB DDR3 and slot2 2GB DDR3. What is the highest upgrade for processor (I think I can afford i5 only) and RAM that I can get? I don't know much about PC so I hope all your help is greatly appreciated. May god bless you all. Thanks in advance.