Halo and gta4 are on the top of the list, yep.
If i were you i would check out the following games:
The orange box. 5 games for the price of 1 - 3 halflife games (single player only), team fortress 2 (online only), and a game called portal which is kinda hard to explain so i'll just link a video review of orange box that includes an explaination:
Mass Effect - If you've ever played Knights of the Old republic it is similar. It was made by the same people. Its a shooter/tech-magic action style rpg.
Call of Duty 2 - not quite as many goodies as CoD4 but it was one of the top shooters on the system before CoD4 came out.
Gears of War - Excellent game. Chainsaw = tehpwnz
Rock Band - fun but kinda expensive. Great for parties and shit like that.
Marvel Ultimate Alliance: Fun party action rpg. If you go to a place that sells used games (gamestop, EB, funcoland, etc) you can get this with forza 2 in the same disk for a low price. Forza 2 isn't bad if you like realistic racing. *shrugs*
And one last one that is free that you can download off of xbox live: Aegis Wing
Edit: yea bioshock and deadrising are fun too. very interesting games.