If you want a very good Rig for 3delight Rendering, I’d say build a Dual Processor Workstation. (Xeon or Opteron) see https://www.avadirect.com/Tower-Computer-Workstations
But that will cost you a bit more than a consumer desktop.
The more cores, and the more theses cores powerful are, the quicker you’ll get render finished. That’s all.to be known
You could plan to build a 2 x 16 Core Opteron or 2 x 6 core Intel, or whatever, it’s up to you
I’d advise beginning with 2 x 8 Gb Ram and extend further then if needed
Professionnal Graphic cards have better drivers to handle OpenGL You could buy one, but I don’t think that is so much usefull for DS or Hex and a good one is much more expensive and you should be ready to pay the price too. I’d say a descent Consumer Card with >2Gb is enough
The main difference in Pro cards is that there is an additionnal cost for a partnership for Certified Workstation Drivers for some applications and there are only a few case on Big Softwares (mainly Autodesk products or Industry CAD softwares like Catia, ProEngeneer, Autocad, Solidworks…) where you would see a difference in handling the mesh. So Take the gamer’s card you want.
Here is a chart to give you an idea or OpenGL performance http://www.tomshardware.com/reviews/geforce-gtx-780-ti-review-benchmarks,3663-12.html#