This is THE deep learning hardware guide that helped me build my machine:
You need a second GPU if you want to train/iterate faster or if you use bigger models. So this depends on your use case (as does the GPU RAM).
64 GB (system) RAM should be fine, but also depends on your exact use case (I have 32GB RAM and this was never the bottleneck so far).
Enough CPU cores and a SSD are really necessary to get the data as fast as possible to your GPU(s).
All in all, your setup looks reasonable from a high level (without checking the compatibility of the parts or other details). 