I currently have a GTX 560 Ti and I am planning to SLI but the other pci-e slot says pci-e 2.0 x16(x4 mode).. and the current one I am using says pcie 2.0 x16 only. What will happen if i installed another 560 Ti on the pci-e x4 mode.
here's my mobo.. http://www.asus.com/Motherboards/Intel_Socket_1155/P8Z68M_PRO/#specifications
Don't do it for three reasons:
1) A graphics card in the second slot will run at X4, bringing down the performance of both cards.
You may actually do no better than a single card.
2) Because of the close proximity of the cards, the top card will have a heat issue. The bottom card will restrict the cooling air to the top card.
3) With two cards, you will be exposed to microstuttering.
I have the same P8P67M-Pro mobo, and it is a good one. Early on, I had thoughts of such a graphics upgrade path.
Really, it would be better to just sell your current GTX560ti and replace it with a stronger single card. Assuming you have the requisite 700w psu to run sli, you also have the psu to run any single graphics card made today. Even a GTX690.