Hi ,
Check whattomine.com
Rx 470s are the most profitable gpus as of now.
For ethereum mining, a 6 card rig would give you 7 dollars revenue and 5 dollars profit approximately, considering elec cost as 0.10 cents.

So 5x 30days = 150 dollars profit
150x12 = 1800 dollars profit per year.

That is considering , the price of ethereum is at a low at this point. When it rises , profitability is more.

Im mining with 5 rigs. And i have custom vbios settings whuch pull less watts per card. I have nothing to complain:)