NVidia GTX 660Ti SLI Performance and Overclocking @ Ninjalane
 

NVidia introduced SLI as a scalable way to increase video performance and it does so by adding more GPUs to an existing system. Depending on the GPU you can link up to four of them together with the sweet spot resting somewhere between two and three.

http://www.ninjalane.com/reviews/video/gtx-660ti_sli