If you can't access your Xbox console and have a burning need to download that Call of Duty map pack right away, you can purchase the item on your computer, and your Xbox will automatically begin downloading it when you next connect to Xbox Live. So you'll always be able to download stuff from Xbox Live, even when you're away from your 360. Thanks, Microsoft!
Each download from Xbox Live tells you how large the item is, and the range is huge — individual songs on Guitar Hero games can be as small as 14MB, with albums going all the way up to 1GB for Metallica's Death Magnetic. Large mission packs, like Oblivion's Shivering Isles expansion or Star Wars: The Force Unleashed Jedi Temple Mission pack are of a similar size, with Xbox Live Arcade games being very small in size, around 20-50MB.
Multiplayer gaming on Xbox Live uses up a variable amount of data per hour, depending on the game — killing zombies with a friend in Call of Duty: World at War will use up a lot less bandwidth than a 16-player Halo 3 death match, for example. Each game will use up a different amount, so there is no real way to say how much data you'll use up, but it would be a good idea not to play online if you're approaching your download limit.
Be very careful when playing online and downloading Xbox content because you can easily push yourself over your download limit without realising it — downloading three demos on a whim one afternoon could set you back over 6GB.
One way to quickly secure a game is to make extensive use of the Friends List feature. By adding friends and those you enjoyed playing with, you can quickly create a list of gamers who (hopefully) share a similar taste in games as you, giving you more of a chance to play your favourite game online.
You should also look out for events hosted by Xbox, and gaming sites such as GameSpotAU. GameSpotAU regularly hosts Games Nights for a variety of different games and consoles, which allows you to play a fun game and maybe meet some new people of similar tastes online.
Unfortunately for you, yes. If you connect to Xbox Live with a console which has been mod chipped, and Xbox Live detects it, your console will be locked out of Xbox Live permanently. This is not done by your gamer tag — this is done by Microsoft communicating with your console to see if you have valid firmware and other software. If you don't, you're out of luck — your console will be banned by the Xbox system.
If you're using a modded console, you will be permanently locked out of Xbox Live
(Credit: CNET Australia)
A gamer tag is the Xbox equivalent of a forum handle, username or nom de plume. It allows you to gain achievements, personalise your console and create a friends list, as well as having other people add you to theirs.
If you want to access your gamer tag from another console, simply select "Retrieve gamertag" after pressing the Xbox Guide button (the large circular button in the middle of your controller) when not signed in. This should allow you to enter your gamer tag, and Xbox will download the relevant information from Xbox Live.
Access your gamer tag by pressing the Xbox Guide button (Credit: CNET Australia)