well I'd say the more cores the better/faster, as that's what's used to do the rendering. Xeon procesors are more stable and I'm thinking if it's for rendering machine that will run on full power most of the time then maybe they would be better (maybe someone else knows). Graphic card is not doing anything while rendering - would only help you in displaying loaded models, but it doesn't speed up rendering times.
Ram - it all depends how big are your models.
ps> saying that GPU doesn't do anything for rendering, SW2016 SP2.0 will have SW Visualize, which is old BunkSpeed - which i think might have option to do CPU, GPU or CPU/GPU rendering - but someone else would have to confirm.
for the fastest rendering, you need as many cores/threads as possible, so if you can find a multi cpu box thats the way to go. ( i dont think you get mutli i7 cpu motherboards, only xeon). gpu, we nearly bought the K4200 recently, but for about the same money we got the newer series m4000's, which have more ram. we have noticed fewer crashes since, so dont skimp on the g card. 50% of my work is drawings, and performance is way better with the new card. regarding ram, we tried 32gb on our machines, and despite being win7 x64, the os never reported using more that about 6 gb, so it seemed unnecessary spending more money on the ram (but we only load small assemblies). theres my 2 pence...