The option is to get the X4 860K vs i5 6100, which is $40 cheaper than the Intel. And decent Intel motherboards which are actually worth as an upgrade path over an i5-6100 start at around $100 (he has to get at least H170 board with good VRMs). For the current case, selling that FM2 board will get him how much? $20? $30? How much extra for DDR4? What would be the FPS difference when used with an RX 460?
At this point, spending $67 or less for an X4 860K is much more preferable than i3+mobo combo, because he already has the motherboard. That is budget SSD money, not to mention being more than enough for an RX 460. That CPU is even in the recommended list of this forum at that price point.
Just get the AMD, its a good match for that GPU for dirt cheap. If you want to build a proper gaming PC down the road, the least I would recommend is a quad core i5 with a decent motherboard, not that 2 core HT crap with the cheapest board you can find.