Left 4 Dead is a fun game, that's for sure. Don't listen to the elitists who say that it's only good on PC - yes, they've got better longevity to it, especially given Valve's history of updates for their Xbox games (read: none) but it's still a great game. If you can pick it up cheap or rent it for a week, you'll do good. And I've been having plenty of fun with it offline, too, so Live Gold isn't really needed. You'll only ever want to play with your friends on it anyway, and for that it's better to get them to just drag their consoles and copies to your house and play System Link instead - you'll have more fun that way, it'll be less laggy, and when they be dicks by moving the portable generator to the top of the elevator in No Mercy, you can just reach across and slap 'em for their troubles (or just be smart and remember to take an oxygen tank with you to blow up and move the thing)
I got Guitar Hero: World Tour for Christmas and I'm enjoying the hell out of it. I've only really played a bit for GH2 and never owned any of them, so for me it's a fresh and novel experience. And it's one of those few games you can play in front of a crowd of complete strangers and get them excited about how well you're doing - no-one's going to applaud you for grinding in an RPG, but hit a few notes in GH and you're in.
The FPS are all pretty much the same. See thing, shoot thing, find bigger thing to shoot bigger things with, try to get thing, win game by achieving thing. Halo 3 is the average of them all, so get that and you'll have a mildly enjoyable experience that quickly fades into nothingness.
And RPGs are very subjective, so do your research on those. I'm playing through Eternal Sonata right now and I'm loving it - others will probably recommend Mass Effect or something but I honestly found it all rather bland and boring.
Oh, and my Xbox Live Gamertag is Aaronicon, if you want to add it for some bizarre reason.