With Nvidia cards, I believe the VRAM size has to match for SLI. Dual cards will not double the amount of VRAM, however.
2 cards will be ~80% faster than a single card in most games that are SLI aware. (That's all modern games) So in that regard, it would be faster than almost any single card you could buy. But while micro-stutter has pretty well be eliminated with dual cards, it still raises it ugly head occasionally.