Definitely get The Orange Box. I'd have just brought it just for the amazing Portal but you get Team Fortress 2 and pretty much all the Half-Life 2 series bundled with it too: Half-Life 2, Half-Life 2 Episode One and Half-Life 2: Episode 2. Five quality games in one can't be bad, can it?
Another game you might want to add to your list would be Elder Scrolls IV: Oblivion -- the Game of The Year version will be released soon which contains the official expansion pack, The Shivering Isles and the Knights of The Nine bonus pack. You could probably pick that up for a decent price about now.
Gears of War, too. Can't miss that. Tom Clancy's Rainbow Six: Vegas, too. Burnout Paradise looks sexay, too. Umm... Tom Clancy's Splinter Cell: Double Agent if stealth games float your boat. The multiplayer's cool any way. Not to mention it's being sold for like $9 on Amazon and eBay now.
Another recommendation would be Command and Conquer 3: Tiberium Wars, then you can play it on Xbox Live with me.
Good luck!
