Depends on your tastes, of course. I'm assuming you like shooters, since you're getting Gears of War. Here are my suggestions:

--Call of Duty 2 - Should be cheap now, and the campaign is really awesome. Also, good for splitscreen with a few friends.
--Call of Duty 3 - The campaign is somewhat boring compared to the first. Not as good. But the multiplayer, with 24 people online and no lag to speak of, is absolutely amazing. Really, really fun.
--Burnout Revenge - It's cheap now, and a damn fun racing game. Way better than NFS or any of that other stuff.
--Oblivion - Duh. Unless you dislike open-ended American RPGs, in which case shy away. Or if you're afraid of it sucking away your life.
--Dead Rising - You might want to wait on this one, since it's been out awhile and will probably drop in price soon. It's a fun game, though.
--Halo 2 - Still one of the best Live games.

When you get your 360 (make sure you go with the Premium) buy a Gold account. And then go onto the marketplace and download the demos that are available. From everything I've read, Ninety Nine Nights, Just Cause, and Enchanted Arms are not worth buying. F.E.A.R. is fun, but short enough to beat in a couple days if you rent it. Not worth 60 bucks.