There's a mouse around here who run's a pair of 560's on a Corsair 620w.
Haven't seen invisik in a while, used to run a pair of GTX260's on an Antec 500, may still be.
Decent power supplies can run almost anything ( within reason for the unit ) , just not for as long as if they were running with a 50% load on them.
( loud booming voice )
Let's go to the review
http://www.guru3d.com/article/geforce-gtx-560-ti-sli-re...
"System Wattage with GPU in FULL Stress = 449W" ( Power measured from the wall socket, on an overclocked, water cooled, extra bling system )
1200w psu ( can't remember which ) let's say 90% efficiency
449*.9 = ~400w
That would be an 80% load on a 500 watt psu ( during the generic test that simulates a game and won't be disclosed for fear of a conspiracy ), kind of high, but not outside of a decent psu's capabilities ( as long as it's power is where it needs to be )