They don't keep the models going indefinitely; if you are planning to SLI in the future don't buy a card that has been out for a while, expecting to buy another in six months or so. It won't be available.
... And you can SLI it with another card, later! :D
But you don't need SLI unless you have a large monitor.
7 series nVIDIA has some extra stuff that the 6 series doesn't.