http://www.xbitlabs.com/articles/vi...f6800u-sli.html
http://www.xbitlabs.com/articles/vi...00u-sli_37.html
Conclusion
So after a long interval we see another attempt of the promotion of a technology that joins several graphics processors to increase the performance of the graphics subsystem of the computer. We have tested the technology developed and implemented by NVIDIA Corporation, and our impressions about it are rather ambiguous.

NVIDIA Multi-GPU SLI: Myths and Reality
Let’s consider it an established fact that the SLI mode does produce a considerable performance gain, up to 100 percent, or twofold, in some cases. NVIDIA’s developer team did a very good job and had brought all possible latencies and time losses associated with synchronizing two graphics card to the lowest minimum possible.

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. In other words, the multiple GPU technology from NVIDIA has a high efficiency, but lacks the universality peculiar to the Video Array system from Alienware, for example.

NVIDIA’s innovation also proved to be most fastidious about the installed cards – only two identical cards from the same manufacturer would work together. We couldn’t make two GeForce 6600 GT from different makers work in the SLI mode, despite their having identical characteristics and the necessary connector.

This is not to say that NVIDIA’s SLI is bad or defective. That’s just “childhood diseases” that every new technology suffers from. Well, even the nForce4 SLI platform itself couldn’t boast maturity at the moment of our tests – the ASUS mainboard worked with a beta BIOS, and with a beta version of NVIDIA’s drivers for the new chipset. This couldn’t but tell (and it did tell) on the stability of the new platform. Beyond doubt many drawbacks now peculiar to NVIDIA SLI will be eliminated sooner or later. There will be more supported games and the Multi-GPU rendering mode will work more correctly.