Typically, using 2 cards will let you get around a 30 to 50 percent boost in performance. This is JUST for games though, and only at high resolutions (I'm not sure how high you have to get it. )
As with anything, your monitor has to be able to output these resolutions too. You also have the options of running things like a dedicated physics card etc. The main downfall is not all games support dual carding.
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Chrono1219