I think the AMD RX 470 could be a good card for the Etheruem mining as long as the memory width is 256 bit.

not better than the 480 already, scaling between the two should be very akin, so i see now point in buying one unless you want a low budget build

The 470 has few cores so that it can save some power consumption as there is no need for the 2300 cores.

The 470 is good for the ethereum mining. But it is not good for other algorithms. The ETh mining will stop within a year.

That is right. It is difficult to decide if we should invest in mining now. The hashing rate is too high at present.

If you do not have existing graphics cards, it is better not to buy new ones now unless you have very cheap electricity price.