You should be fine with that PSU:http://www.guru3d.com/article/geforce-gtx-470-480-review/13
"GeForce GTX 470
On your average system the card requires you to have a 550 Watt power supply unit. We recommend at least 35 Amps in total available on the +12 volts rails (accumulated). A second card requires you to add another 250 Watts. You need a 850+ Watt power supply unit if you use it in a high-end system (1 KiloWatt recommended). That power supply needs to have (in total accumulated) at least 55 to 60 Amps available on the +12 volt rails."