New X-Box -- Suggest me some games?!

Hey all!
My mom recently said she was going to get an X-Box for the family, Which really isn't bad since my sis likes the same games i do so we can co-op =/

Anyways, I was wondering if you could suggest some games?
Like FPS (First Person Shooter) or RPG (Role Play Games)
Or any other Action/Adventure.

I'm already going to get Left 4 Dead 2, And Tomb Raider: Underworld (We are ordering it online and it comes with 2 free games)

Along with Halo 3, What are some others that you have played or heard about that look/sound/are epic?
(And if possible under $30, but they can be over if they are worth it. )

Thanks!

-Bob