Ethereum forum, they made a calculation and said most GPUs won't be able to mine Ethereum due to DAG limit. Only 4GB and higher will work.
Ethereum forum: https://forum.ethereum.org/discussion/4394/extending-mining-through-the-use-of-sli-for-large-dag-files#latest2GB DAG mid-2017, take it from someone who should know and did the calcs (dlehenky)
If I only had one ETH for every "miner" sitting on the sidelines waiting for PoS to arrive....
The DAG will be 3GB by the end of this year. But the mining will be not possible before then. It will be PoS.
The DAG size increase 200MB every 6 months. So the 280X should be enough to mine for another 12 months at least. But PoS will come before then.