Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
My friend plays it on his 13" (1.8ghz) without a problem on low settings.

If you get the 2.0ghz i7 upgrade, I don't see why you couldn't run it at at least medium and get 35+ fps since you also have a smaller screen res.
 
The fan might be a bit annoying. Consider you go from "never-haring-a-thing" to the fan being on all the time while playing.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.