For Video Cards a minimum of 4GB is recommended because, in general, it is enough to handle 3-4 dressed figures with hair and an environment. 2GB cards may or may not handle 1 figure with clothing and hair (roughly 50/50).

Having said that, I have, personally, run out of RAM on a K6000 (12GB of Ram) with one figure clothing and hair because I pushed the limits on tessellation for displacement.

IMHO, the best bang for your buck is the GTX 980Ti, because it is roughly $630 compared to the Titan X at $1050 (Roughly) or the M6000 (($4000-$5000) is generally faster than the M6000 and only 3% slower than the Titan-X.