Last I tried SLI and CF (4 years ago), the FPS maybe was higher with two cards, but the games didn't feel smoother for it. In fact, with two cards, staying above 60FPS was more essential than it was with one card. Is this still the case?
How well does SLI scale with a uber-modded skyrim?
I find SLI does scale much better then it used to. Back in the day when I had SLI GTX 280's I had much more issues with SLI then I do now. Really the only issues I have now is if the game will support SLI, all the ones that do run very well. I also think for the money SLI GTX 970's are a great deal, considering it is not much more then 1 GTX 980.
You could also always get one GPU and if you find it is not enough purchase a second one.