Congrats Cheapy. My recommendations?
1) Try out Halo lan, or PGR2, Crimson Skies, any game that has system link you should give a try. If you got a basement/room with multiple plug outlets and room for 4 tvs, give it a whirl. At the moment I link up two systems with two tvs and have five or six people over for Halo LAN, and the action is non-stop for several hours.
2) Get live. For live get Counter Strike, R6 and one good racing game.
3) Game Recommendations, out of my current 72 XBox games theres a few I think are worth every penny and are must haves. Most of my games I got cheap and don't consider must haves as is.
Halo (which you already have)
GTA Double Pack
DOA3 (Atleast at it's current price)
Links 2004 (I know this sounds silly but it's an awesome golf game)
Top Spin
KOTOR
Metal Slug 3 (Maybe when its a bit cheaper though)
Ninja Gaiden
Otogi
Panzer Dragoon Orta
PGR2 and RSC2 (Take a pick, or get both)
Shenmue 2
Steel Battalian (although that's for the rich ass)
Morrowind GOTY
Riddick
Multi System Games: Splinter Cell, SC

T, Max Payne 1+2, Prince of Persia, Soul Calibur 2, Burnout 2, Beyond good and Evil, The Suffering and Silent Hill 2. Everything else might be worth a purchase, but all these games worth it no matter what imho.
And as always gamerankings is your friend when looking for other titles. But you know that
4) Now you should always re-consider your multi platform picks. Not just for better graphics and frame rate, but perhaps additional content.
Edit: Damn I actually put this as a new topic by accident for asecond.