In order to get the best results, you should use a card with an identical part number to the one you currrently own to run SLI.
You cannot mix and match different series video cards in SLI. For example, a GTX 780 cannot SLI with a GTX 770, nor would a GTX 660 and a 660Ti work together.
Both cards must have the same amount of memory. You cannot mix memory amounts.
You can SLI between different speed versions of the same series. For example, a GTX 780 Superclocked and a GTX 780 should be able to run in SLI. Just make sure to run the faster model as your primary card in the SLI setup.