1) 485M SLI = two GTX 485M graphics cards. 6970CF = 2x HD 6970s.
2) Most laptops chassis don't have the physical capacity to have twin graphics cards installed and keep them cooled.
3) If CF/SLI scaling were perfect you'd get up to double the graphics performance. Actual performance differs due to a number of factors - like a specific game, the graphics card, the LCD resolution and the CPU being used.
Yes / If you 'need' to play Metro 2033 or Crysis 2 @ 1920x1080 resolution @ ultra settings. And you want a good shot at playing games released in 2013 on ultra settings.
Maybe / If you can afford it.
No / If you play COD: Black Ops or older games like CoD 4 @ 1366x768 resolution and your budget can't handle a GTX 485M price tag.