i'm running the r9 fury with 250 watts a piece for 470 hashes each.

If you're buying brand new get RX480 and you can get away with a 1000/1200 watts PSU running 7 for the same hashrate (330*7 rx=2410 ....  400*6 R9= 2400). I think the 390x will run at around 225 a piece so you may need at a minimum 1600 watts and be careful with the 15 amps breaker. Even if the the breaker is 20 amps be careful with the wires on the wall as they may melt.

price wise you will have a better deal with with rx480 ($1400 versus $2400) for the same end result. If you wait another month for the rx580 release they will be even cheaper.

Remember mining is foremost about economics.