If money is no object then the 680 should see you well into the future, a 660TI would give you more than enough power for todays games and most that come your way in the near future. Tbh the 670 would be the card of choice as there is nothing it can't do that a 680 can.
MSAA and FXAA are different methods of removing jagged edges from 3D graphics, MSAA uses the GPU itself along with a large chunk of video memory and as new techniques have been introduced to more modern games, MSAA has needed more grunt to do it's job.
FXAA on the other hand uses shaders instead of GPU and Memory making it a more optimal choice in some games, in many cases making the difference between playable and unplayable framerates.
The new 6xx series of cards now have TXAA which I believe is similar to FXAA but better quality.
Generally SLI will give you a 90% boost in framerates if the drivers are written optimally for the games, it can sometimes take a while for these drivers to be released. The only situations I can see SLI being of any use is for when you want 120fps for monitors that support 120Hz which are generally used for 3D Glasses. The other situation would be for 3D games over multi monitor setups. Otherwise a single graphics card will do nicely.
Most monitors are 60Hz so anything above 60fps are wasted on these as the frames aren't even shown on the monitor.
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