Oh your psu is fine a 500 watt could run that but it would push the limits you have a 700 watt you'll be fine And I acutely have done this before i started mining I bought a 5870 and mined with it and my gtx 470 I couldn't get it to work for the life of me I couldn't get both drivers to work properly.
Damn, I thought the one I had was 750W but just checked and it was only 650, still be good with a 100W cpu?