I am following Rohan's terrain tutorial. my viewport bogs down somewhat when I make changes and/or rotate view. I have an NVidia GTX950, cpu is i5 4670 w/16 GB.
I know this subject has been beaten to death over last several years but with advancements in cpu and gpu, so for the short term so would I get more performance with a GTX1070 or faster processor and 32GB ram? both right, but I don't have funds to do that at one time.

Both could do with an upgrade. The GPU does the displacement, zoom out to see the viewport speedup, and the terrain tools are heavily multithreaded, when manipulating the terrain settings. 4 cores isn't that much these days.

You can reduce the load on the GPU with Display Options/Effects -> Displacement Level

the viewport uses openGL, if your scene is complex a gpu with large amount of ddr6/hbm2 ram is gonna be helpful, quadro card tends to have better viewport anti-aliasing and stability. Rarely do you need more than 32gb of ram to do casual simulation, but if it's a very large scale sim it's likely you'd be working at studio where they would provide you with proper equipment. But as of now, a GPU is a better investment, not only can it boost your viewport responsiveness, it also enables you to render with popular gpu renderer and get fast result.

Thank you both for responses. I will upgrade my video card then and save up to get a new mobo/CPU/RAM