1. You'll be able to run nearly every game out there. It's just that games that can't use SLI will only access one card. As a general rule, if a game will benefit from SLI, it will be able to use it. My two 650ti Boost's make the games I play MUCH more enjoyable than one did.
2. Nope. Neither does any other consumer card configuration. Consumer cards aren't really made for that, particularly those from nVidia.
And, like ITF, I would always choose a single powerful card over two less powerful cards if the money works. On the other hand, one more GTX 680 will give you some very nice gaming horsepower, and you're likely to be able to get one for FAR less than a single card that can out-perform 680 SLI. Then you can enjoy better...