For a single GeForce GTX 560 Ti graphics card system NVIDIA recommends a 500 Watt or greater system power supply with a minimum +12V current rating of 30 Amps and at least two 6-pin PCI Express Supplementary Power Connectors.
For a single GeForce GTX 570 graphics card system NVIDIA recommends a 550 Watt or greater system power supply with a minimum +12V current rating of 38 Amps and at least two 6-pin PCI Express Supplementary Power Connectors.
For a single Radeon HD 6950 graphics card system AMD recommends a 500 Watt or greater system power supply with a minimum +12V current rating of 36 Amps and at least two 6-pin PCI Express Supplementary Power Connectors.
The Silverstone Strider Series SST-ST56F maximum combined +12V current rating of 40 Amps (480W) and with its two 6-pin PCI Express Supplementary Power Connectors is sufficient to handle a system with any one of those three graphics cards you have listed.