I suggest using 8Gb for Ethereum mining. When Dag file size increased, Polaris cards will be affected.
This config will be good enough for your Rig:
Depending of the number of GPU...
For a RIG of 6 GPU you can use:
Mother board: H81 Pro BTC R2.0
CPU: Intel Haswell Processeur Pentium G3260 3.3 GHz 3Mo Cache Socket 1150
But to be simple CPU is not use at all during mining process so a small one is enough.
For disk it gonna depend of the OS. For a linux USB key 8g is enough but for windows SSD 40g is require.
Memory 4G is enough for linux 8G is better for Windob.