It would appear that 650w would be fine considering the power draw of double 4870s (4870 has a similar power profile to the 560) is about 400w. The i5 draws less power than most of the quad core i7 test machines as well.
there's a reference link, but even assuming TOTAL system power, including ALL components doubled because of your SLI setup, which it most surely will not, you'd still be fine and at about 520w of power draw.
That 650W power supply can actually handle approximately 800W power draw. It will be fine with your SLI setup.
I have the SeaSonic X-Series 560W with one GTX 570 over-volted and overclocked, plus a 2500K over-volted and overclocked, 12GB of 1600 RAM, and a water cooling loop for the CPU. When gaming, my peak power draw is less than 375W. I could add another GTX 570 and still be fine.