It's not the wattage of the power supply that determines whether a power supply unit is capable of properly powering a system.
For a modern system you need to look at the power supply unit's maximum combined +12 Volt continuous current rating and it should also have very good to excellent electrical noise, ripple and transient voltage suppression (i.e. high DC Output quality).
For a system using a single GeForce GTX 560 Ti graphics card NVIDIA recommends a minimum of a 500 Watt or greater power supply with a +12 Volt continuous current rating of 30 Amps or greater and with at least two 6-pin PCI Express supplementary power connectors.
Select from reputable brands like Antec, Corsair, Seasonic and XFX.