What should I upgrade to improve my rendering speed?

Solution
You need double the ram and double the threads. 16gb of ram and an i7.

As far as GPU that completely depends if the rendering program you use does GPU parallel processing to utilize CUDA cores.
If it can then a GTX 1060 (even 3gb model) will make a good impact. If the software does not then even a GTX 1080 ti would not make a bit of difference.
You need double the ram and double the threads. 16gb of ram and an i7.

As far as GPU that completely depends if the rendering program you use does GPU parallel processing to utilize CUDA cores.
If it can then a GTX 1060 (even 3gb model) will make a good impact. If the software does not then even a GTX 1080 ti would not make a bit of difference.
 
Solution