If you are contemplating crossfire in future,you need to get an 800-850 watt power supply.Unless you are overvolting your cpu and gpu a lot,850 watt psu would be sufficient. Since you got the reference 290, which runs hot and loud when overclocked, when you add your second 290 which should be reference as well, you don't need to overclock them as the performance in crossfire @stock clocks would be more than enough to get you 60+ fps @1440p in almost all the games.
Also you should look at getting 1440p 120hz single display like the qnix 2710 instead of going multi monitor.Get an 850 w psu like corsair hx 850 or antec hcg 850 psu for now and add your second 290 down the road.
Also you should look at getting 1440p 120hz single display like the qnix 2710 instead of going multi monitor.Get an 850 w psu like corsair hx 850 or antec hcg 850 psu for now and add your second 290 down the road.