2x 8800 GTS or 1x 8800 GTX

Hello all,

 Looking for some input from you folks. I'm putting together a new pc for gaming and am undecided if it's better to go with 2 8800 GTS cards in a SLI config, or to just go with one 8800 GTX and add another later if needed. Are there many situations where SLI being enabled will help me with performance, or will I get just as good with most games these days with just the one 8800 GTX?

 Appreciate any input.

Thanks!