hmmm i really dont know, it were me id buy a SLI board anyway, even if i dont do SLI right away for me it would nice to have the possibility that i can boost performance later on for cheaper than buying a whole new card.
6800GS is a good card all on its own, it should last 9months. but i reckon 10x7 in the newer games will be called for. i know i had to start doing that on my 6800GT. obviously 2x6800GS would be pretty good, but then you may as well of bought a GTX for the same money.
it depends how you like your games, since my 6800GT i have come accustomed to hi res, AA and AF....so i must have those which is why i have 7800GT's in SLI.
as always id say get the best you can afford at the time, and dont bother waiting for newer stuff down the road, because in this game you'll end up waiting forever. (it only makes sense to wait if the new thing you want is like 1 month away)