In my opinion SLI is only something you'll want to invest in if you wish to max out the eye candy in your games. However once a new graphic innovation appears, cards which dont fully support this in hardware are goin to slow up some. Therefore you'll pretty much want to have SLI straight off the bat - grabbing that second card further down the line will be a false economy. Even if you could find a dirt cheap one it would still ring in at least the same cost as your next mobo.
If you can get a good price for your Ultra then I'd be tempted by the 7900 GT otherwise cross your fingers and hope there are no yield problems with the new generation of cards.
Good point on DX10.
I suggest that you stay with the 6800 Ultra until the next generation of cards is released that support DX10, then make your decision.
6800 Ultra is still a pretty good card; just hold out for the next-gen stuff. I'm debating what to do about my X600; newegg has an X800 for $72 after rebate which is looking really good right now. But first, I need to earn some $$$; I have $6.50 in my coin jar; the rest of my money is locked up in a CD.
i agree with Lbax, stay with the current 6800 ultra until the new dx 10 cards come out and get 1 or maybe even 2 then for sli. they are coming out with faster cards so often that it shouldn't be too far off for the next generation.