If you plan to keep expanding that system, then hit at least the 1300, if that is as far as you are going to take that one, then 1000 or 1100 should be fine. These things do vary, the amount of heat generated and not pulled off the system will cause a more inefficient power use.
Even the local electric makes a difference, there can be a difference of up to 40 volts at the wall socket in some place up or down, 10 or 15 volts is no biggies and most things work fine or make internal adjustments, but once the drain is increased then major things can happen. It is better for a computer to have a complete power loss for a short period than to have too little power coming into the system.