All of the Halo games are great. I still play the shit out of Halo Wars, its a real time strategy game. I play it with jaSSon on here. Also if you want to get sucked into a game, try out Fallout 3 Game Of The Year edition, Fallout: New Vegas Ultimate Edition, and the Elder Scrolls games. The Orange Box is cheap and includes Portal among several other games.
If you don't mind used games, gamefly.com often times sells off rentals that they no longer need. Tons of games in the $10-$20 range.
Live is worth having, but I decided long ago to remove my credit card info. If you don't pay attention its super easy to rack up huge charges for buying points for dumb shit like movie rentals. If you pay attention to the deals website, its easy to pick up a year subscription card for ~$36 which is WAY better than the ~$11 a month they charge to go month to month.