So I recently built my new PC and it was the first time I built one. I was bench marking Metro last light and got low fps with physx on and I was thinking I should SLI. I have an i7 4770 k 3.50 ghz 4.0 turbo gtx 780 ti 3gb evga 8 gb ram. What card should I get to SLI that can help my machine run Metro high fps with physx on?