RX 480 has faster (8000 Mhz effective) but narrower (256 bit) memory than the R9 290 and R9 390 that gives it overall slightly better memory bandwidth than the R9 290 (5000 Mhz effective at 384 bit) but slightly worse than the R9 390 (6000 effective Mhz at 384 bit).
don't know what are you arguing about but R9 290/390 have 512bit width memory bus