well i personally wouldnt consider a card with less that 4GB. if you are going to render complicated scenes i would recommend 32GB of system RAM.
I am using a GTX 970 4GB Phanton and dont have any issues. I also have 32GB of system ram which i upgraded a while ago as i was not able to render a complicated scene in 4k
hope that helps
Custom built W10 Pro 64 bit, Hazwell Intel 5820, Asus X99 MOBO, 32 GB DDR4 Ram, EVGA nVidia GTX1080 Founders Edition, 512GB SSD Primary HDD, 16TB Storage, 27" Samsung 4k Display. Wacom 27QHD, Oculus Rift CV1