There is a little benefit when you SLI two weak cards and that is you still have two weak cards and not much improvement Just a little but like said above. If you have the money to buy two cards.
Then you have enough to buy one bigger card. I have two 550 Ti's in SLI and it does better than one 550 and it is better than 6770 on down. I can play a lot of games but none of the top games of today.
With out having a lot of problems. The games I can play are RPG and Strategy type games. So I would buy a GTX 560 Ti or a HD 6870 or higher. That is with in range of what two cards would cost as to one.http://www.anandtech.com/bench/Product/290?vs=330http://www.anandtech.com/bench/Product/291?vs=290http://www.anandtech.com/bench/Product/539?vs=541