Even 6gb vram won't be enough very soon, ETHpow will start where ETH left off so don't expect any lower DAG size, if you want to invest in any graphic cards its better to buy those with 8GB vram memory.
Before you invest in any 8gb cards you should at least see what happens with ETHPOW. It could be a big flop and could be completely dead in 3 months. Even if it’s not dead then the profits will be horrible because at current valuations of $40 a coin. It will have a very small miner reward.