[RickRuski]

One thing you will have to consider is your power supply if you go to sli. Just remember that the power required by two cards will double that required by one. Yes as Grathos says some of us have Sli working but it is not officially supported at the moment with CoD. Luthier say they broke Sli support just before release and haven't fixed it yet. I for one seem to have got it to work reasonably well with almost double the frame rates of one card.
I have EVGA Precision tool monitoring my cards performance and it shows V/ram usage at nearly 1gb on both cards so it looks like V/ram is a big factor at the moment (my cards are gts 450 1gb). Everything else seems to be working as it should in Sli with temps, % and frame rates all about equal with each other. Putting 2 1gb cards in Sli unforunately doesn't give you 2gb of V/ram it still stays at 1gb. I'm hoping that the new graphics patch will make a big difference and my advice is wait until that comes out and then see how your system performs before making a decision.