Searched there, don't see anything about MW3 and the Air. However there has been talk in this forum in the past on how the Air performs will COD games.
And neither of the above links posted have the information you are looking for.
I built a system for my friend for $360 that can max MW3 I called it the 360 Project, the affordable gaming PC.Personally I'd recommend just paying the $200 for one of the Black Friday Xbox deals for all my gaming. That's how I play.. it's nicer on a large TV than a small computer IMO. It is also much cheaper than investing $1,000+ in a decent gaming computer.
Even Steve Jobs said Microsoft had a nice product with the Xbox. I think it is the only good thing they ever made, lol.