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).
 The RX 480 has 12.5% MORE compute cores (2304 vs. 2048 for exactly a 9:8 ratio) at quite a bit HIGHER clock rate than the R9 390 and even more so than the R9 290.
 RX 480 and R9 390 are both PCI-E 3.0 cards, R9 290 is only PCI-E 2.0, but that has little or no measurable effect on most mining.

 The RX 480 is NOT "close or a bit less than a R9 290" but in fact is a superior card across the board except ONLY for memory bus width

 Might also want to pay attention to the R9 290x vs the R9 290 as they have the same memory system but the 290x has the same 2304 cores that the RX 480 does