Hi there

I would go with i7,but all depends on budget there

AMD FX range is not bad there,they're like or love multi threaded application,in rendering AMD CPU should be bit faster and in single threaded applications like is OpenGL there i7 can be faster,but all depends on how well application is optimized

Regarding the GPU,I would wait with purchase GTRX1070/1080 as right now CUDA is not optimized for lots of renderers(Octane 3 is still not compatible with GTX1070/1080) and I think most of the companies waiting on new CUDA Toolkit 8 due this most of SW is still not compatible

Regarding the GPU,I would go with 980Ti without the question or older 780 6GB,this GTX780 6GB is fast as my Titan X in rendering with IRAY or Superfly,I've done few tests

i7 platform is better for many reason,you can upgrade later on to Xeons,like I'm running E5 2683 V3 which is 14 cores/28 threads and this CPU is slow in single threaded apps but in rendering,this CPU will destroy my i7-5820k which is OC to 4.4GHz

There is lots of pros and cons,but you need to know what budget is for this custom build

Hope this helps

Thanks,Jura