To answer the Halo 2 question - - you have to buy the Xbox 1 version. The Xbox 360 is backwards compatible with certain XBOX 1 games, Halo 1 & 2 included.
Kameo - I recommend it! Very fun, good looking game.
Madden 2006 - Nice graphics, missing features. If you already have 2006 for another platform, then don't bother with it on the 360, otherwise get it.
Tiger Woods 2006 - Same as madden. I haven't played this one online much though, still just building up my personal golfer.
Condemned - One of the games I wanted the most when getting the xbox, and I have to say I like it. Not as much as I thought I would, but I hear it does pick up in chapter 5 which is where I am now, so maybe it'll revive me.
PGR3 - I'm not a big racer fan, but I have heard so much about this series. It's fun, but I'm not a big racer fan like I said so I'm having trouble getting into this game. Burnout is more my thing I think, but I'll keep trying.
COD2 - Great single player. Great graphics. Online play is kinda stinky. Save game bug is annoying, but IMO one of the better launch titles. You should own or at least rent this.
PDZ - The opposite of COD2. Single player stinky, multiplayer great. Lots of options on multiplayer. If you're an online freak, you'll like this.
NBA2k6 - Gorgeous. Good online play with leagues.
I also rented and beat GUN. Fun game, not next gen, not worth the buy -- rent it. Its just a port so don't expect much.