buy cod5 , or wait a little bit and get cod : modern warfare 2

You should download some demos from Xbox live if you have it. I find its very helpful in choosing new games. You can also view videos of games to give you a better idea of what you are buying.

resident evil 5. definitely an addictive and challenging game. split-screen co-op is the best!!!

You can never go wrong with Call of duty 4, that game is incredible. Also, you could wait for Brutal Legend to come out that game is great. You could try out the demo and see for yourself. Also, Need for Speed Shift, they really got their act together and pulled together a fun game. But, like people have been saying try out demos and see what sucks you in.