NVIDIA SLI is supported by this graphics card should you be interested in pairing a second GeForce GTX 550 Ti. However, under Linux the benefits of SLI are not nearly as great as Windows, due to the lack of demanding OpenGL applications / games. The most intensive game engine on Linux right now is the Unigine Engine, but we are still waiting on many Unigine-based games to be released. Rage also is not expected on Linux until 2012 or any other id Tech 5 titles that would benefit from multiple GPUs. There are some OpenGL workstation applications for Linux that can benefit from NVIDIA's multi-GPU rendering technology, but then you would be after Quadro hardware and not a mid-range GeForce product. There is also no SLI / multi-GPU support in the open-source Nouveau driver at this time. If you are considering multiple GeForce graphics cards and Linux is your primary platform, you are much better off going after a single high-end graphics card instead.
The EVGA GeForce GTX 550 Ti is a graphics card I purchased mainly for another NVIDIA graphics card to be used in the large upcoming NVIDIA Linux driver comparison between the binary driver and the open-source Nouveau stack. This will be the equivalent article to the September comprehensive Radeon 28-way graphics card comparison under Linux using the Catalyst and open-source Radeon Gallium3D drivers. The EVGA graphics card carried a $140 USD price-tag.