With the recent Xbox 360 price drop, retailers are going to be competing for your business. That means kick ass deals for you. Amazon has a very co-op friendly deal going on right now - purchase the Xbox 360 Pro with a 60GB hard drive for the new $249.99 price; and you'll get one of the following co-op games for free: Halo 3, Fable 2 or Halo Wars. You choose and you also get free shipping!
If that's not your thing you can get the Xbox 360 Elite console for the new $299 price with one year of Xbox Live Gold for free!