Definitely get Rainbow Six Vegas. The SP is awesome and the MP is worth the buy alone. You do get Dirty Disc errors but only after playing for a long while and even then it's not that annoying considering other games get it while you play Online for long period of times too.
Gears of War is also a must of have just cause of how fun it is to play and the sweet graphics. The MP is fun but the only drawback that I think it has is that you can only have 4 on 4 matches and I prefer to play with more people to make it more fun.
GRAW 2 is also a sweet game with a solid SP and MP. These three games are must haves if you have LIVE in my opinion.
Crackdown is a fun game but after you beat it it pretty much lost it's appeal to me. I'd say rent if you get the chance or if you have Gamefly, keep it for a month and half and then send it back. Most people only kept it so they could get into the Halo 3 Beta so I'm sure you can get it cheaply on Ebay, EB, or Gamestop.
Splinter Cell: Double Agent was a fun game and I enjoyed it alot. I thought it was easier than the previous ones but it was still fun and worth the buy. If you liked the MP from Chaos Theory then you'll like the MP in Double Agent.
Hitman: Blood Money is also a good stealth based game and I enjoyed the customization and how you could slip into disguises compared to the Splinter Cell series.
If you like racing forget Project Gotham and go directly to Forza Motorsport 2. It's currently what all my friends are addicted too and it's the best realistic racing game out there if you liked Grand Turismo. Personally I prefer Need for Speed: Carbon since it's more arcade like and just more fun to me personally. Especially since you get to pick if you want to start out with Exotics, Tuners, or Muscle cars and the story.
Guitar Hero II is also must have especially with the downloadable content that the other consoles don't have so you can buy song's from the previous Guitar Hero or exclusive's.
As for games that haven't been released yet I would suggest:
Splinter Cell: ConViction
Army of Two
Tom Clancy's End War
Need for Speed ProStreet
Grand Theft Auto IV
Call of Duty 4: Modern Warfare
Battlefield: Bad Company