Goto youtube. Look at videos with the games you play driven by either the i3 or i5. Make your choice.
For most games the i3 is enough to balance a 1050ti or Rx 470. Both of those video cards should eat any game they see at 1366 x 768.
If you are money limited likely you are better off with the i3.
I would get a good 1080P monitor, they are very cheap now, then see if you can afford the i5. I'd rather have 24" or 25" of 1080p with an i3 then play at 1366 x 768 on the i5. But that's personal preference, you might prefer your monitor (e.g. maybe its an older 40" or 50" tv and you liek the big screen).