Hardware & Technical Upgrade opinions please!

My current pc, bought in Jan, is i5 4670, Gigabyte Z87-HD3, GTX 770 2GB, 650W Corsair RM . Now it looks as if I may have/want/need to upgrade to go 4k and maybe O Rift (thanks Frontier!). I have never built or upgraded a pc before but would like to have a go. I'm thinking Gigabyte GA-G1.Sniper Z9, Corsair RM Series 750W or ?850W, Zotac GTX 970 x 2, or maybe Zotac GTX980 x 2 if it would make much difference. I may have to leave the double GPU's until nearer Dec, especially if going for the 980's, or get one at a time!. Does it make much difference what make of GPU I go for (Zotac, EVGA, MSI, etc)?

Does anyone have any opinions on this upgrade? I'd really appreciate any help from the experts here.
 
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