Look at the overview of your motherboard
. It only states your motherboard supports AMD CrossfireX and not NVidia SLI.
If you want dual graphic cards, you'll need to use two AMD GPUs. Given that the second slot is only x4, I wouldn't recommend going with a dual card setup. If you want to use NVidia cards in SLI, then you will need a different motherboard; with which you can make sure the two PCI-Ex16 slots will run at x8/x8 or x16/x16.
Personally, I'd just recommend staying with a single card solution, like the GTX 670 or HD7870 (depending on budget and power supply).