Again just remember bottlenecks is a misused and dynamic term. There is no single hardware that bottlenecks another (outside of maybe pairing a 2nd gen i3 with a brand new top of the line GPU)
"Bottlenecks" are application dependant.
In CPU intensive applications, you may find the 3600 may bottleneck regardless as to the GPU.
In GPU intensive applications, you may find the GPU will bottleneck regardless as to the CPU.
It's all about identifying the applications you run, and identifying what performance you want from it, then what is the most logical upgrade from that point.
As a side tip, always make sure you're running a good quality PSU will almost any discrete GPU...