I just wrote a great reply and lost the post... Oh well
My ultimate suggestion is buy used games. They are cheaper, and you can find out what style of games you like without going broke.
Heres a list of MUST play games imo...
(quick list)
Halo 3
Mass Effect 2
Gears of War 2
Batman Arkham Asylum
Bioshock
Battlefield Bad Company 2
Fallout 3
Mass Effect 2
Assassin's Creed 2
Call of Duty 4
Left 4 Dead
Orange Box (Half-Life 2/Portal)
Forza 3 (Great game,what else is there to say)