power supply have three main power outputs. 12v/3.3v/5.v the 3.3 and 5v you dont have to look at. it the 12v line that used by the cpu and gpu.
when i talked about split rail i was talking about vendor taking the 12v 600w and splitting it into two 300w rails or more. on these units one rail may not hold up a new gpu. most newer power supply use one rail system. with power supply also have to look at the amps rating of the 12v line to see if the vendor not fugging the numbers of there unit.
amps*12v=watts. so if the vendor rated this unit 12v line near 580w the amps rating should be 43 amp. one thing if you add up the cpu and gpu max wattage useage and it close to the max of the 12v rail then bump up to the next size.