You creat a name when you start the xbox and then you can add gold subscription to get live. I'd reccomend a Wireless thing. It's better then the ethernet leads. Silver limits you to things and Gold limits you to everything.
Halo 3 is about £25 I think. I'm not sure :P
And just a tip; when you're setting up your xbox go to run then type cmd then type "ipconfig /all" and then you will get your ip adress and gateway and everything to set up your settings to play online.

