SLI is running TWO cards together as one. So no, you can't have SLI enabled without two cards. But regardless, I can't imagine you ever needed two 8800 GTX cards for quite some time now...
unless your single card would be schizophrenic.. no, it cant run in SLI.
however, you can run a single 8800GTX in a SLI motherboard without any problems (so you could add one later on if you want to)