I would take a closer look at Newegg and be sure to get one of less expensive models. The other choice for under $300 would be the GTX 560 Ti 448 cores model, which would be an even better upgrade for your 560.
A good 500 watt PSU is sufficient, so you are good with that Corsair unit.
Well, you don't run CX500 with 560 SLI. I know the 560 SLI only won't take 400 Watts of power (sorry for not saying this before), but the system wattage will be needing ~500-600W when everything is fully stressed. You get a better PSU, because running CX500 is too risky and even maybe will not suffice. I know you can get some TX650, but you don't have the future proof of the TX750 which has 4 PCI-E connector.