He can expect a heavy bottleneck. He'll probably lose more than half of FPS as compared to using an i5 4670k. For instance, if an i5 4670K +GTX 970 can get 100 FPS, imagine an FX6100 +GTX 970 will only get 40 FPS. An FX6100 has to be clocked around 4.6 to 4.7 Ghz just to match a stock i5 2500k. I suggest he upgrade the cpu first before the gpu.
10/18/14 at 12:11pm