When building a computer and trying to figure out what power supply to get a lot of times it can be a good thing to look at the requirements of the video card. In this case the GTX 570 has a minimum requirement of 550w for a power supply. In that case I would get a 600w psu to give yourself a little room instead of getting just what the minimum is. The differences in price are not all that much and you can easily get a 600w psu for $50.
CORSAIR Builder Series CX600 V2 600W ATX12V v2.3 80 PLUS Certified Active PFC Power Supply
$69.99 and a $20 rebate makes the final price $49.99.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
You can also use the power calculator.http://images10.newegg.com/BizIntell/tool/psucalc/index.html