It's not the idle power you have to worry about, it's when the system is under a heavy load.
Just the other day I was running furmark on my PC. The UPS I have it plugged into provides 700 watts of power, but when my 750 Watt PC powersupply was pulling more power than the UPS was rated for, it started going into alarm. I had to shut down Furmark before the UPS gave out.
I have a very simular board to yours. I have the XFX 750i extreme edition motherboard. It only has (2) PCI-E X16 2.0 slots thought.
You can calculate your system power using this site. http://www.thermaltake.outervision.com/Power
With (1) GTX 560 ti and your GTS 250, the site shows the system power draw at 565 Watts. With the 570, the number jumps up to 628 Watts, using a 90% load. I just used it for my setup, and with (2) GTX 460s and my slight over-clock, the system would require 600 watts. My 750 Watt power supply isn't what I'd though it was and from what I've read on the internet, this unit needs to be replaced soon since it starts to show problems once is passes 650 Watts.
Here are a few shots of my PC and the results for your system with a GTX 570.
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