Theres a bridge that you use to connect them both one card does the top half one card does the bottom half of the screen it pretty much doubles performance
current SLI does not have a huge performance benefit for current games, but upcoming ones will most likely see a huge benefit, especially with all that AA and AF
u make a good point that it dosent have much of a preformance boost right now but like u said it will allow u to play with high AA and AF also high resolution with high fps