With DX12, Vulkan etc IMO Radeons are much more the way to go, while 770 are indeed powerful (I only have a 7870 while I love it, am waiting on AMD new offerings) I would say that you would be better off to update to new when available, and in that aspect, wait because both Green and Red are coming with new ones very soon.
If it were my $ and had to have it now, a single 380x if you are rocking up to 1440p(may have to reduce settings slightly) or 390-390x for beyond, obviously want at least a 650w which am sure you have
New Tomb Raider and mentioned DX12-Vulkan will make multi-gpu and lower power usage on nearly all parts I am sure, probably make new ones that can take advantage of the modern absolutely shine, the scaling for CF and SLi in this game is very awesome, so while I never was a fan of dual card as many cases are simply NOT designed for it as mobo makers do not space very well the ports and such very well, and to be able to have proper spacing usually requires a big case for proper size mobo, they really need to get away from a hard socket design and maybe go for more a ribbon style, could SO have better features and cooling ability that way.
