Were you intending to run the MSI GeForce GTX 560 Ti and the MSI N560GTX Twin Frozr II/OC in 2-way SLI mode? It won't work because the the GPU is different between each of those cards. If you actually were to install an MSI N560GTX Twin Frozr II/OC along side your GeForce GTX 560 Ti all you'll end up with is two independent graphics cards.
A GeForce GTX 560 Ti GPU must be paired with another GeForce GTX 560 Ti GPU (graphics card manufacturer can be different).
A GeForce GTX 560 GPU must be paired with another GeForce GTX 560 GPU (graphics card manufacturer can be different).
Is your power supply unit able to handle two Geforce GTX 560 Ti in SLI mode if you decide to go that route?
Does your power supply have a combined +12 Volt continuous current rating of 45 Amps or greater?