GeForce GTX 570 in SLI
A second card requires you to add another 225 Watts. You need a 850+ Watt power supply unit if you use it in a high-end system (1 KiloWatt recommended if you plan on any overclocking).
For each other card (3-way SLI) that you add, just add another 225 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