The GTX560 offers better performance, but it also sucks more juice. Make sure your PSU is up to the job.
Also, as good as their mobos are, Asus is the only video card brand I've ever had fail on me within hours or days of first use. Any others that failed did so after years. I prefer XFX, HIS, or MSI for graphics cards (yet I wouldn't risk a MSI mobo, especially if I planned to OC).
Will a seasonic SII 520w would be enough to pair it with core i5 2500. (i am not going to overclock)