For a system using a single reference clocked GeForce GTX 560 Ti graphics card NVIDIA specifies a minimum of a 500 Watt or greater power supply that has a combined +12 Volt continuous current rating of 30 Amps or greater and that has at least two 6-pin PCI Express supplementary power connectors.
Adding a single reference clocked GeForce GTS 250 changes this to a minimum of a 650 Watt or greater power supply that has a combined +12 Volt continuous current rating of 45 Amps or greater and that has at least three 6-pin PCI Express supplementary power connectors.
If any over clocking is being performed then you need to allow for more power capacity to handle that.
If you have a SilverStone Strider 560W SST-ST56F, it has a combined +12 Volt continuous current rating of 40 Amps and has two 6-pin PCI Express supplementary power connectors.
If you have a SilverStone Zeus 560W ST56ZF, it has a combined +12 Volt continuous current rating of 38 Amps and has two 6-pin PCI Express supplementary power connectors.
Both of the power supplies mentioned are insufficient in terms of +12 Volt continuous current rating and number of PCI Express supplementary power connectors.