Rx 480 4gb overclocked can do in v9.1   235h/s on 180watts wall, i have seen the 480 8 gb can do 245 h/s and i think they do this on the same watts or even a little bit less watts as the 4gb i'm not sure but 8gb can get a little bit more hashes/s as 4 gb. I think the biggest different is the price in my country the 4gb 50 till 80 $ less than 8gb