NVIDIA Multi-GPU SLI Technology: New Approach to Old Ideas - That said, we could agree with NVIDIA's claims about a super-high efficiency of SLI technology
if it were not for one problem. SLI does not work with all games. In many gaming applications there was no performance gain whatever or there even was a negative effect from enabled SLI. In some cases enabling SLI made the system unstable, or a game just would not start up. Another drawback of SLI technology is its dependence on the optimizations for each particular game in the ForceWare driver. If the game is optimized for SLI, the performance gain can be huge, but if the game is not in the database, there's a high probability of your getting a very small speed bonus (far from the promised 70-90 percent) or none at all.