Any current card is incredibly better than the GT 120 that you have. RX-560 is half the price of the RX-580, but the latter has better macOS support, have HEVC hardware decoding with Mojave, probably we will get hardware encoding working too.
RX-560 have some driver support problems with 10.13, RX-580 had only with 10.12, none with 10.13.
RX-560 do not need supplemental PCIe power, RX-580 needs a dual miniPCIe 6-pin to PCIe 8-pin cable.
Hey mate. I installed the card but it looks like I need a power cable. Front page of the booklet says you need power. I'll have to go tomorrow. Do you know if a standard 6 pin to 8 pin pcie cable will do? I'll get googling. Thanks AGAIN