Mousemonkey :
That is something that you really need to sit down and think about, 2x 660Ti's will give you the same or better performance than a single 680 in games that support SLi (and most games do these days) but will leave no room for upgrading but then if you go for a single 680 and decide in a years time to go SLi will you be able to find another one new or will you have to get a second hand card that may have been OC'd to within an inch of its life? And what about cards that have been released in the meantime? A single one of them may be a better upgrade than running two of whatever you decide to go with in the first place. There are other considerations but I hope you get the gist.
Yea, but look at his budget. Two 660ti's would cost $550-660+ (not to mention higher cooling/psu costs if he doesn't currently have enough of either), he is in a $300-400 range budget. If you are spending over $500 on cards, obviously you will be doing SLI/Crossfire (unless you pay for some massive overclocked 680/7970 or go for a 690/7990). In the OP's price range though, a single card will more often than not be better, unless you already have one a card to pair it with, find a smoking sale or have a friend give you card or sell you one cheap.
Also, the OP did ask for advice on what to do, I gave him mine. He can take it however he wants to. However, you quoting it in the way you did is arrogant and it is slightly insulting, since you gave no argument to support your position (I'm not going to loose sleep over it or anything). Basically that's like saying that I'm completely wrong, I should know better and you can't believe that I would even suggest that.
Also, nowhere did I say that he should never SLI/Crossfire, in fact I said just the opposite. I said "in the future you can crossfire/sli if you so choose". I did not say "100% of the time or its a fact that sli/crossfire is bad". There are always going to be different scenario's that different advice will apply. But based on the OP's price range and him not mentioning his previous card to crossfire/sli, I think (along with many other in this thread) that a single 670/7950/7970 is the way to go. No one has yet to post an SLI/crossfire build that would be viable in his price range and be better the previously suggested single cards.
And as for your 660ti sli setup, that is a good setup, yes it will beat a single 680. And if the OP said his original budget was $500-600 you bet I would say go dual 660ti's. But he didn't and that setup will not apply.
The problem I have with going crossfire/sli right off the bat (on lower budgets like this) is you (probably) are going to be spending the same amount of money and getting similar enough performance. But with the single card solution you still have the flexibility to upgrade when the next gen of cards come out and put your performance on par with the next gen of cards. Doesn't a pair of 570 look pretty good today? Which you can still buy 5 different brands/models of 570's on newegg right now. My friend just SLI'ed his 480's for something like $150 4-5 months back when Newegg had their final "480 sale" (he already had the psu/cooling in place for that). Granted if you do wait for too long you may miss your opportunity, but you normally have a year or so after the next gen cards come out.
The strongest argument you made is that one of your cards fail you will still be up and running with one. That's probably the most valid reason to SLI. Personally, I think, the flexibility of skipping a generation buy picking up a second card down the road, normally at a far cheaper price, out weighs that perk.
Again SLI/Crossfire has its place, and I am in no way against it, but I still stand by my original advice unless you can find something to better convince me otherwise. I will tweak my original statement for you, so you can understand it better. My advice when starting a fresh build or upgrade *where you do not list your current card as a possible crossfire/sli candidate* and are *on a limited budget* I would go with a single card over sli/crossfire right off the bat. Unless you are able to crossfire/sli high end cards, *within your budget* as that provides you with maximum crossfire/sli flexibility later on when you may wish to upgrade.