Right now I have a Samsung Khronos 700Z5A (look at my sig).
Tasks or games to play: Battlefield 3, Metro 2033, Age of Empires III, Call of Duty MW3 and Crysis 2 /w dx11+HRT.
How much of a performance gain will I see from buying the Lenovo Y580
The 660M will add about a 30-45% performance increase, and the fact that it's Kepler and the new Ivy Bridges are out, it would add a decent performance boost in battery life as well.
It's your choice if that's worth the grand.