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.
Yes they are weak, you run into more probllems and its a waste of money, why bother when you can get a much better single card solution and then be able to crossfire or SLI it later, anything under a 560ti or 6950 in dual card operation in my opinion is not worth it.
Grab yourself a decent card first go unless you are on a tight budget or have an older computer which cannot take the stronger cards.
I just bought a second 550Ti for SLI cuz NewEgg had a sale, wound up like $80 after MIR. People like to sit and hate one SLI for weaker cards. Think of it this way.
SLI of weaker cards is good if you can only afford one weaker card at first, then a few months later have enough to buy a second card to SLI.
If you choose between 2 Lower cards for multi GPU OR a single higher end card, go with the higher end card as you can later buy a second one of those for multi GPU system. 2 Lower end cards will require more power and better case cooling (as they put out more heat) than a single higher end card.
SLI was initially made for only top of the line cards, then enabled on lower end cards because it allowed people who couldn't afford those higher end cards to get comprable quality in games.