Well, Halo is the best game ever made, especially if you're planning on playing with another person. Crimson Skies is also very fun and easy to pick up and play. Both of those are $30. If you like Star Wars or RPG even a little, Knights of the Old Republic is a no brainer. If you like racing games, Project Gotham 2 and SSX3 are both solid choices. If you're planning on getting the Xbox Live online service, grab Splinter Cell 2.
But if you like shooting games even a little, I'd make Halo my first purchase.