I don't know what the requirements are for SC2, but my Lenovo laptops do everything I want them to do (graphics work being the most intensive) for about $400. I would not be surprised if you could get a decent gaming rig for $500-600. Lenovo are not lightweight, though if you're looking at it as a gaming platform you're not likely to get away with a superslim superlight model anyway (my friend who is a teacher has a very slim one he does things like AoE and CivV on though, so I may be wrong in that).
:edit:
http://www.newegg.com/Product/ProductLi ... ame=Lenovo - Lenovo, specifically designed to be gaming laptops. I was a little conservative on price.
http://www.newegg.com/Product/ProductLi ... emoryMax=8 as a non-gaming counterexample in the price range I was talking about. NewEgg is the bomb if you don't have a Fry's where you live.
And yes, stay away from Mac if you want gaming, or even to just eat non-Ramen meals for the next year. Fuckers are way too expensive and locked-down.