I do want to say too, anytime you buy a new game, the very 1st thing you should do is download at least all the free content before doing anything else. If patches or various fixes are found, MS will put them out free of charge and it makes the whole Live experience so much more worthwhile.
At times, new maps may be developed which will make the game more fun, bigger, just more playabililty for the $$$ really. There may be a charge for this type of download $5.00 or whatnot. IMO, it just makes the games more enjoyable and allows that $50 to go further. In some racing games for example, you might be given extra circuits, in shooters, you'll get more maps, in a game like Dance, Dance, Revolution, you're given more songs. It just keeps the developers working on games even after they're already out. You will NEVER have to buy any of these though, but........you'll not be able to play against gamers who did download it. That's just a choice you'll have to make for each game as content is released. Sometimes we download it and sometimes we pass, depending on how often a game is played.
Register your Xbox and make sure Microsoft has your email addy when you do. Anytime a download is made against our credit card, I get an email from MS saying they'll be charging our account. I love this feature because my son likes to sneak without telling me and he knows he can't do that now! We have 2 Xbox's and let me tell you, paying that $5.00 to get it onto both makes me angry. In cases like this, I make him decide which system he wants the content on and he simply can't play the game on the other Xbox.