Yes it will work. Although like said above, the same exact card is ideal.

Go for it. The faster GPU will just downclock to pair up with the slower one. No biggie. The difference between the OC and Top version is only a few MHz.

Just make sure that they both have the same amount of memory.

Chino's last point is the only salient one; There is absolutely no problem except the VRAM size has to be the SAME. As long as both cards are 2GB or both 4GB no problem.

It will be trivial I suspect to get Afterburner and clock the card you have to the level of the other. Or simply let the GPU boost feature run the cards at separate boost speeds. Keplar cards can boost to different values on latest drivers so performance differences in cards are less of a limitation...the slower card will not necessarily limit the other to its speed through the boost clock.

The top card in the SLI will always work harder so just put the OC model in top slot and then between the extra load for it and the ability to boost higher you will hardly notice a different even if you don't want to OC the other card.

Memory has to be the same, beyond that any make and any model of the same series.