There is no difference in Skyrim between a 2500k at stock settings and a 2500k overclocked. It's more than capable of maxing the game out on stock. At those settings, it'd be difficult to notice the difference between it and a Phenom II.
Actually there is a difference, skyrim is a very cpu dependent game and overclocking can seriously help out with framerate. Most cases it wouldn't be noticed however seeing as though skyrim locks itself at 60fps anyway.
Check out these articles for proof of cpu scaling and whatnot: http://www.tomshardware.com/reviews/skyrim-performance-benchmark,3074-9.html and http://www.techspot.com/review/467-skyrim-performance/page7.html
@Gtyjrocks - the second build on that page with the 2500k and 560ti should be able to ultra the game fine or play ultra with high shadows. Not entirely sure about the second build but the processor is a bit old now and doesn't really hold up to even a high end i3 which would be around the same price point (I think it would be anyways). Check the links above about the cpu's and how they affect skyrim framerates for more info!
@MichaelWesten - the GT640m isn't really a strong card and I'm not sure it would be able to handle high settings, let alone ultra (or high for that matter) with mods. The screen res isn't that high though so it might let the 640 give you high settings, not really an expert on laptops though so it's hard to tell!
After checking lenovo's website, the Y580 (http://shop.lenovo.com/SEUILibrary/controller/e/web/LenovoPortal/en_US/catalog.workflow:category.details?current-catalog-id=12F0696583E04D86B9B79B0FEC01C087¤t-category-id=AC523278A4F13F27A84F5F5622D1AC7A&action=init) would certainly give better gameplay with its GTX660m but it is a little pricier (only 50 bucks going by their Y series overview page though) and it has a 15.6 inch screen instead of a 14 inch screen which makes it heavier and obviously larger so it would be a little less portable...
Like I said though, I'm not really sure with laptops so it's probably best to just take someone else's advice!