Ok well here's the other live games I have
MechAssault - Oh man this is the original live game. I would play this for hours with my friends and it was great. Loved playing games where one team would be the weak fast mechs and the other be the giant slow powerful mechs and having the weak ones always win.
Crimson Skies - I haven't played this that much because of the my RS3 addiction, but what I did play of it was great. I loved starfox on N64 and the multiplayer was fun when playing with my brothers and friends and CSkies is like a extremely updated version of that game.
Project Gotham Racing 2 - I only played this a little, but it's a very pretty game and a lot of fun too. It's definitely the best racing game I've ever played.
Unreal Championship - This one was ok. It's kind of like Halo but on a ton of sugar. If you like fast paced FPS games this is the on for you.
Rainbow Six 3 - Oh man, this is THE best Xbox live game with SC:PT and MA close behind it. I've racked up almost 300 hours of play time because it never gets old. You can play coop or adverserial and both a great. Coop is a lot of fun with your friends because you can plan out what your going to do and attack different areas for multiple entrances and use your weapons to make your missions easier. The Team Survival is IMO the best gametype of RS3. I love it because the games are short and sweet. Most last 5 minutes tops unless you have campers. 8 on 8 games are great, but I really like playing 4 on 4 or 3 on 3 because those games i can make full use of the remote charges which is one of the most embarassing ways to die. I also like TS because you can just get in a game and play a few rounds in a short amount of time which is great because I can get a few games in before school. It also has a pretty good flow of downloadable content with 5 new maps since it's release. So if you want a great live game, this is the one to get.
I left out SC:PT because you already have it. I'm looking forward to these games though. Rainbow Six 3: Black Arrow, MechAssault: Lone Wolf, Halo 2, Doom 3, and Goldeneye2 .