When talking about 2 midrange beating 1 high end, the thing to rememebr is that there are exception to that, like 2 GF7800GTs beating 1 GF7800GTX-512 for less money, or 2 GF8800GTS beating a single GTX or Ultra for the same money, but it depends on the situation, because there's no clear cut rule for that.
When I referred to midrange SLI vs single high end we refer to 7600GT SLI vs 7800GT(X), not 7800GT SLI vs 7800GTX. At least to me there is High end (Includes, but is not only top of the line), midrange, and low end/mainstream. To me the high end of a graphics card line would be for example ALL 6800 based cards of the Geforce 6 Series (Includes Vanilla, GT, Ultra, GS...), the midrange would be the 6600 series on all it's flavors (Vanilla, GT...), and the low end are 6200 cards, now this can all vary depending on how severely crippled a card is compared to the rest like midrange labeled car dangerously close to being a low end, as far as I know those might be isolated incidents like the 6600LE.
My point is, even if you get SLI 8800GTS over a single GTX you will most likely get better performance on some/most titles but it will not always be the case, however getting 8600GT on SLI to do the job of a 8800GTS will not get you anywhere near close the performance of the aforementioned. (Which is what I was referring on my first post)