The theory goes that the cards work in parallel giving you twice the processing power and memory band width. Of course, driver and scaling issues mean that sometimes you do not get double
Just to add to puma...
Most of the time you won't see double the frame rate because the scaling won't be +100% for most games. It is usually only the big triple A game titles (mostly First Person Shooters) would be do that. Most other games would not scale to give that much of an increase on performance.
And then there's matter of CPU bottleneck. When you have extremely fast graphic power at hand, there are times which the CPU won't be able to keep up, thus unable to unleash the full capability of the graphic cards.
For example, a pair of GTX770 in SLI themselves are capable of delivering 100fps in a game, however if the CPU is only capable of pushing 60fps, the actual frame rate in game would still be at only 60fps, rather than the full 100fps that the SLI 770 that's actually capable of.
The most common situation for that to happen is that games are not written so use more than 1-3 CPU cores. One of the main reason why the Intel i5's 4 cores CPU is having an edge over AMD's FX-8 8 cores CPU for gaming at the moment is because the i5 is pretty much as fast (if not faster than) the FX8 even when it is 4 cores vs 8 cores both with all their cores fully utilised. The biggest weakness of the AMD CPU is that in games that use only 4 cores, they effectively loses 50% of their processing power for that game, whereas the Intel i5 would still have their full 100% processing power available for that game; in a game that use only 2 cores for example, the Intel i5 would lose 50% of their processing power, but the AMD FX-8 would effectively lose 75% of their processing power (6 out of 8 cores not used for the game, rather than 2 out of 4 cores not used).
It's a bit outside topic I know, but it's worth bearing in mind.
