The memory speed must be 1750 MHz or higher. 1500 MHz is crab.
I have a couple of ASUS cards that were originally 896&1500. After unlocking to 1024 and changing the straps they give ~14Mh/s ETH.
However for sure I agree that if you have several options it's better to take 1750 ones.
its enough 2gb ram or should be 4gb ?
For ETH you need at least 3Gb