The GTX 1070 which has 8GB of vram, is currently rendering faster than my Titan X (maxwell) system in DAZ public build beta. Once that is optimized you will see very impressive results. I saw some good deals on below $400 GTX 980 TI 6 GB cardz, which also have similar performance in render to my Titan X but for the $ and the additional vram, And likely additional optimization, I would put my money onthe 1070 or 1080. The hybrid versions of those cards would be even better. Avoid the founders editions.

check out Sickleyield benchmark discussions you can see a lot of different cards results.