As the owner of 2 8800 GTs in sli, I feel I am qualified to answer this question. And the answer to your million dollar question is...
:: drumroll ::
They perform predictably well. They're not going to be blow your mind "OMG THIS IS AWESOME" but you should be extremely satisfied with their performance. I run a custom config, but my settings average out to about high (excuse me, "gamer") and I run 1600x1200, no AA, 8x AF. I had fraps do an fps benchmark of just me playing the first level for 15 minutes and I averaged 52 FPS. The Ice levels were as much as 10 fps slower, but well within the playable range.
As far as the scaling goes, I ran the original crysis GPU benchmark without sli and got 29 fps average. With sli I got 51, so that's about 75% scaling you're looking at.
This is all with 2 GB RAM, 780i, E6750 @ 3.4 ghz