You do not need a 1200 and 1000, thats overkill. Full power draw of a 6x280x system is 1665watt. Get 2 decent PSU's to cover that much wattage with a little overhead.
Math dictates that but onhand experience says depending on the card a pair of 1050's for the XFX, Visiontek's would work, but a 6x toxic R9 280x's hashing at 806 a piece were near 305w a card, thus requiring 1200w per each set of 3 ... and there was a bit of a stretch for the MB/CPU combo.
Yes you can get by with less but you are REQUIRED to undervolt and/or adjust powertune/core/mem clocks down from stock.