Assuming you have a cpu that isnt drawing more than 115W, you technically could run two of these with 650W. I do not think it's smart, since each card is 250W TDP and typically cpus are close to 100W, so that's 600W of power. If you have a less than spectacular power supply, say less than 80 plus gold, then im not sure I would risk running it.
I would get a bigger power supply, about 850W would be best. Im using the evga 850 G2, and its fantastic. Plus its only $140 on newegg. Fully modular as well.