Well 500w should be plenty for that system. Also heads up there isno need to upgrade to an i5. Why? The 660 is slightly underpowered nowadays and it is a better match for an i3. If you were to upgrade to an i5, the RX 480 is the best GPU option to upgrade to.
Also you need a PSU upgrade even though wattage is enoguh,miss because that CPU is crap and is not reccomemd end for a rig like yours. It does not feed clean power to your components. You should never have cheaper out on a PSU. Spend some money and grab an XFX TS 550w or antec green 550w PSU for 60 bucks.