NVIDIA SLI certification ensures a quality user experience by putting components through a rigorous series of tests including diagnostics, benchmarking, burn in, and card interoperability. Only components that pass NVIDIA SLI certification can be called "NVIDIA SLI Ready". For a list of NVIDIA SLI Ready graphics cards, please visit the following URL:
When choosing SLI ready graphics cards, as a general statement for SLI compatibility, the graphics cards you are wanting to use in SLI need to share the same GPU (i.e. Geforce GTX 670 + Geforce GTX 660 will not work but Geforce GTX 670 + Geforce GTX 670 will work), Memory type (DDR3 vs DDR5), and same memory volume (1gb vs 2gb). GPU Core and memory clock speeds can vary meaning you can mix stock with factory overclocked cards.