The recommended wattage of 500 covers bad quality psus and dells psus are actually pretty decent. As you have said, the 560ti requires 30A so as long as the 2 pcie connectors are on different cords it should work.
The Dell PSU should work. Contrary to popular believe that all OEM psu's are crappy (majority), dell PSU's can actually do what their rated at. Although they are not that efficient, they won't blow up.
I still recommend you replace it later when you have money just for efficiency and the fact you don't need to keep your A/C on that much (less efficient = more heat).