Can I run SLI with 2 different model video cards?
Unfortunately SLI will not function with 2 different model video cards. They each will have to be the same model in order for SLI mode to work, however they may not have to be the same brand. When yoking two GPUs together, it is best that they be as similar as possible. This is a limitation by design.
If card-A is a manufacturer's overclocked or superclocked edition, and card-B is a standard edition, the faster of the two video cards will down-clock to the same speed as the slowest card in your system to help ensure stability.