How important is the processor now that rendering can be done by GPU?

If you carry out most/all of your rendering with a high-end GPU is it important to use a mid- or high-end processor?

Where is the processor used? I assume moving around the viewport etc. is carried out by the GPU?

An example: if you built a new PC with a celeron/core2duo processor and a GTX 580 GPU, did all of your rendering in cycles with cuda and built all of your models in wireframe mode, would the cpu cause a bottleneck in your workflow somewhere?

I’m sorry if this is a naive question.

Nice question. GPU will do most inner loop of integrator, ray/triangle intersect test, Monte Carlo core calculations, node evaluating. All other still CPU only, as all data preparation every frame: converting all objects to triangles, BVH building, some data refactoring to fit GPU needs. Anyway, you definitely need fast CPU, as it scale ALL other tasks, as smoke /other simulations, modeling, composing, anything. GPU scale only fraction of overall work, and some complex scenes can not fit in GPU and MUST be rendered in CPU.

GPU rendering is rather limited to previewing at the moment. As soon as you have a scene that uses any large amount of geometry or large textures you’ll be right back to the CPU.