When is SLI actually good?
Answer: When running 3DMark
For most games, not all though, you will see almost no difference and you will have a new layer of pain on top of that. We're talking flashing shadows, microstutters, crashes, game needs you to disable SLI to play, random "application will not start" etc. The list is long. You are better off waiting for the next generation if you feel your GPU is underpowered, something I can't really imagine being the case. I'd say something else is troubling your system, not that your card is underpowered.