I'd recommend a 1050 Ti, its about $150 but if thats too spendy then a 1050 will hold over just fine. If you overclock the 1050 Ti you should be able to handle that game at ultra at 60fps at 1080p but it will handle what you want just fine I'd recommend EVGA over any other manufacturer.
Even with an I7 6700K and a GTX 1080, i drop to about 45-55 fps in very dense jungles. Ark is not very well optimised, so keeping above 60 fps is very hard for any card.
Go for a gtx 1080 4 way sli maybe then you will be at around 60 if you can even start the game up with 4 gpus and 4 way sli is complicated trust me I had 4 way there was about 30 problems and games i could not play due to not being able to use all 4 gpu's lol so 2 way sli 1080's for $1600