EVGA 850W G2 Supernova (rebranded superflower), phenomenal PSU and cheap to boot.
It'll support all your Xfire needs when the time comes, and is a great PSU to have in the meantime anyway
285 is fine, ASUS or Sapphire are the brands to go by AMD wise.
However, it appears Nvidia and AMD are planning new cards shortly (GTX 960 and AMDs RX 3XX series)
Might be worth waiting for those (It could be a couple of months though)