Originally Posted by ocpokey
SLI 570s at full stress, GPU consumption 410 Watts at the wall (348.5 to system). You have p-l-e-n-t-y of room left on a quality 650 (your CPU isn't going to consume > 300 Watts).
Really, don't waste your money on 850 watts that you don't need.
From the link you just posted!
GeForce GTX 570 in 2-way SLI
A second card requires you to add another ~225 Watts. You need a 750+ Watt power supply unit if you use it in a high-end system (800+ to a KiloWatt is recommended if you plan on any overclocking).
For each other card (3-way SLI) that you add, just add another 250 Watts and 20A on the 12V rails as a safety margin. What would happen if your PSU can't cope with the load? Here are some pointers:
bad 3D performance
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
PSU overload can cause it to break down