What do you mean by major brand powered by AMD or nVidia ? you already going GTX 580s....
Also i would change these 2 for HD 7970s in CF, almost the same price but with a performance gain to HD 7970s.
1000W Extreme Gaming Series ? Never heard of that brand and a 850/900W PSU will be sufficient for your rig, just get a quality branded one from Seasonic, Antec, NZXT or Corsair.
thats because it's ibuypower but i need to know how to build an sli system.