you desperately need at least another 4gb memory module (that should be your 1st priority)
4gb of ram are just very limiting these days , you need at least 8gb for desktop pc .
you picked a motherboard with low-end chipset so no OC and no crossfire
(even if crossfire was possible the second card would be limited by poor speed of the second slot
due to pcie lanes distribution of this chipset)
best thing to do is to sell your current amd 6870 and buy at least gtx 1050 or (ideally) 1050Ti
http://www.game-debate.com/gpu/index.php?gid=3541&gid2=452&compare=geforce-gtx-1050-vs-radeon-hd-6870
ryzen 1500x can handle even more powerful video cards than 1050/1050Ti
but due to current prices / availability and (presumably)
limited budget i assume gtx 1060 or 1070 are out of the picture .