I've been running SLI for a long time. Started with a pair of 7800GTX's and now I've got a pair of GTX 280's
It's hard to say just what kind of an experience you'll have without knowing what cards you'll be using and the games you like to play. As it stands for me I can play every game I own maxed out at 1920x1200. But there's alway the next new game that demands a little more for all the eye candy.
My advise is 2 is always better than 1. You may not need the power today but there's always tomorrow