I'll throw in a vote for the 360. Fantastic online experience. Yeah, it's about $50 a year, but worth every penny. I'd say 95% of my gaming on the 360 is online multiplayer. Once you get a taste of it, it's tough to go back to single-player.
As for Call of Duty 4... don't even hesitate on that one. By far the best FPS out there. What I like about it is that even though they went modern warefare on this one (instead of WWII), it's still very realistic. No computerized HUD type stuff to pull you out of the game. I don't care for games like Ghost Recon that have all HUD graphics oulining enemies and such-- drawing attention to the fact you're playing a video game. I want it to feel real. On a big HD screen you will lose yourself in CoD4.
Tomb Raider: Anniversary is a nice game if you remember the original. I loved the first couple installments of TR and this is a great updated-throwback to the original.
I'm not 100% sure, but I think the ELITE version is the only version which has HDMI out. So if that is the case, definately go that route.
The extra controller is a good idea if you have friends over. The battery packs are invaluable. If you don't have the rechargeable battery packs you'll be eating through AA batteries like mad. Pick up the charging kit that plugs into the wall (it comes with one battery pack) and then pick up a second battery pack.
Walmart.com : Microsoft Quick Charge Kit Xbox 360 : Accessories