make sure GPU is fully seated, take some RAM out and try to turn it on, take out one gpu and leave on and try to turn it on, make sure all the motherboard cables are completely in their proper socket
I would just try altering my settings or getting extra ram. I have the exact same gpu but 16 gigs of ram and a 6600k. I run this game flawlessly when I'm not in big cities. (I think the r6 and discrd thing is a ram problem too).
Tbh 8 gigs sounds good enough