580 is nvidea right?
its never good to mix ATI with nvidea
but its fine to have difrent cards in the same PC long as there all ATI or all nevida
now most of the time when thay say a 6990 needs 750 or what ever thay mean the hole PC not just the card eg I am runing a 6990 right now off a 450w psu but the psu is only powering the card nothing els
for your 6990 and one othere nice card I would vote 850 should be fine
or you could buy
http://www.pcauthority.com.au/Review/132947,thermaltake-express-power-450.aspxand keep the psu you have