I plan to sell the GTX570 and buy 3 x RADEON 5870 is my PSU with 700W enough ?
Or it will be better if i keep my gtx570 and buy 2 more RADEONS 5870 ? Is 700w enough for them ?
IMO, no in both cases. even if you mean "keep removed from system/PC" about 570 in 2nd case/variant.
but thats strongly depend what EXACTLY you PSU can output on 12+V bus[in Amps]. more likely that you need about 850W or more powerful , PSU.