As of now, January 16, 2017 the DAG size for Ethereum (ETH) is 2.28GB. Why is this important?, the DAG size determines if a GPU can mine a particular currency or not.
The current DAG size for ETH is 2.28GB, which means GPUs with memory size of 2GB can not mine ETH. However, a 2GB GPU can still mine Musicoin, Expanse, Ubiq & Soilcoin.
According to https://investoon.com/tools/dag_size, 4GB GPUs won't be able to mine ETH in 3 years, 6GB GPUs in 7 years, and 8GB GPUs in 10 years.
This calculator is only an estimate of when a GPU may be obsolete, but depending on how fast each block is mined a 4GB GPU may still be useful well past 3 years. So if you're like me and would like to prepare (as much as possible) for the worst, I personally would stick with 8GB GPUs just to be on the safe side. If 8GB GPUs aren't available 6GB GPUs would be sufficient.
Here's a little status update of what I have mined, coming close to 1/2 an ETH, WHOO!!
I ran into some problems here and there. On January 12th, Windows decided to randomly freeze on me for an hour and I decided to diagnose the problem, which gave me a blank screen for 10-20 minutes. I got scared and reverted everything back to the way it was and gave up. Instead of fixing or finding out the random freeze problem I'm thinking about investing in a WIFI Smart Outlet Plug so I can just restart the computer when I'm away from home.
Thanks for visiting.