The AMD A4-5300 is a very poor cpu. Nonetheless, you will see a big boost in gaming performance. I would still recommend you buy a better cpu. The best for the A4's FM2 socket would be the AMD Athlon II X4 760k, which is around 50$ on ebay.
As for which RX 460 to buy, any one is good. You might as well buy the 2GB version since it is cheaper and you wouldn't benefit from the 4GB version. So, just buy the cheapest one you can find and then invest in that Athlon CPU.