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?
Your CPU needs to feed information to the GPU to process it.
The i3 6100 is capable of sending enough information to fully utilise the GPU processing.
By using a GTX970/980/980ti, Information being feed by the CPU won't be enough to maximise the GPUs processing power.
i3 6100 can send information that can maximise performance of GTX 750ti all the way up to the GTX 960.
I am not saying the 970 will cause lag, Im saying it's power will not be used entirely by the CPU, making it a waste of money.