Originally Posted by zelgit
Ok @feznz thanks for your input. So you mean that if one is overclocked and the other one isn't then it will not work? Or do you mean that there is a possibility that, even though non of them are overclocked, they could have different "base" clock?
In nvidia with SLI you should try to match the clock speeds 1:1 between the two cards exactly, for best compatibility and stability. You can try it without it but it would most likely cause games to crash / driver resets / etc, because "the proper way to do it" is supposed to be to combine two 100% identical cards.
So if you have one that's overclocked and one that is not, you would have to either overclock the not-overclocked one to match the other, or down-clock the faster one to match the slower one.
Either way the result would be the same: matching the clocks.
A overclocking utility like MSI Afterburner / EVGA PrecisionX would be useful for this.
It's not 100% a flat requirement to match clocks, as some configurations work without doing that.. but you should, just so everything works smoothly.