Dell Optiplex 790 SFF upgrade

Hello,

I am trying to build my own gaming desktop. It is my very first time building one so I am new to a lot of stuff. These are what I have right now: Dell Optiplex 790 sff, Samsung 8 GB ram, Kingston 480 SSD, Cool master 4o0 watts power supply, AMD Radeon HD 7570 low profile GPU, and i5 3230M CPU. Now, I am aware that the cpu I have will not fit into the socket, and that I need to buy a new one. Which cpu should I buy? Which ones are compatible? Also, is my desktop going to be a decent gaming desktop? Any advice would be appreciated.

Thanks!