Oh, I don't know if you can do that at GameStop, but I've preordered it for a few weeks on Amazon.com, you can probably do the same (and I get promotional discounts for preordering, so each new game is only 40 bucks lol).
I heard AC: Brotherhood was good, but I've preordered Gears of War 3 and Homefront...that game looks sick as all get out. Did you see the videos for it in the 360 Dashboard?