I have a new Area 51 R5 and am considering upgrading the power supply from the 850W to the 1500W unit.  Dell says the part # isn't available and that it isn't in stock anyway.  Does anyone know the part number for the upgraded PSU? Also, is the system pre-wired for more than two GPU's or will I need to purchase additional PCI-e power cables for this PSU as well?  If so, any part numbers for those?