It depends on the game and resolution. If 1080p and a game like bf1 it'll probably bottleneck a single 1070 much less 2 in sli. Even 1440p with 1070's in sli will probably be held back a bit in that game. There's no easy answer though generally speaking the 6400 is one of the weakest i5's. Slow core speed is going to be an issue in several titles. You'll probably want a fast i5 or an i7.