No, you can go up to a GTX 960 with no bottleneck
So are you saying if I add a GTX 970, it will cause lag?
NO!!!
It won't cause lag, the GFX will simply be overpowered and won't be much better than a 960 in some games. All depends on your settings though. If you have enough money to consider a 970, just get a i5 6600 and a 960 instead.