As mentioned, the wattage rating on the GPUs box refers to an entire system, not the GPU itself.
For a single GPU, 550W - 650W PSU is more than enough. It will even allow some overclocking.
For dual GPUs, including a Single card with dual processors, 750-850W depending on the PSUs quality and the type of GPUs.
For example, a Seasonic x750 or Corsair AX750 will run SLI GTX 570s or 670s without issue, amoung others of higher quality builds.
Lesser quality and/or some "Bronze" rated PSUs may
require a move to the 850W to allow more headroom, depending on your setup and power consumption/peaks.
Anyway, Im getting off track here. A good quality 550-650W for 1 graphics card is all you need. And youll have plenty of power for extra hardware and overclocking. If your in this range now, and its a decent PSU, you have nothing to worry about. Happy Gaming!