PGR4 pisses on PGR3 so much it's not funny. It's a return to the golden days of PGR2, especially online.
Earth Defense Force is not only cheap but far too much fun. Co-op lads taking on giant ants, UFOs and robot dinosaurs in a fully-destructable city? Yes please.
If you liked Tony Hawk games then get Skate and be amazed at how much better it is than any THPS effort. You won't be able to go back, the trick mechanic is so damn satisfying.
Halo 3, Bioshock, Rainbow Six Vegas and Dead Rising all need to at least be given a go too.
And you have Xbox Live, right? Pointless having a 360 without it.