You won't have any problems using them in a 3.0 slot they are backwards compatible and you won't loose and performance and ofc you will need the cpu that supports it which is any ivy bridge cpu..just making sure you know that
Yes, your GTX 560 will work in the PCIe 3.0 slot.
PCIe 3.0 is backwards compatible with PCIe 2.0, so a PCIe 3.0 graphics card will work in a PCIe 2.0 slot, but at PCIe 2.0 performance/speed.
Ok, so I need a special processor for PCIe 3.0 to work? Will an AMD Phenom 1090T x6 work with PCIe 3.0?