as already elaborated many times, the SU performance - besides of appropriate modeling - is determined by the CPU speed (single core performance), for a new system an intel Core i7-6700k surely delivery the best bang for the buck.
Investing lots of money in the GPU won’t help much besides for render plugins resp. applications supporting GPU rendering. The brandnew GTX 1060 is a neat graphics card on par with a GTX 980 with a low power consumption (= silent) including 6 GB V-RAM available from 250.- U$ already (Asus or MSI twin coolers are fine).