You can not they are different networks restricted to each console
Yes, as long as one has Xbox Live membership then he/she can play with others. You need to have Xbox Live Gold though to be able to play on multiplayer based games like Halo, COD and what not.
You can play it while signed into xbox live, but you cant play with other people through xbox live on oblivion.
There is no way. PS3 users play on "PlayStation Network". XBox users play on "XBox Live".
Yes.
For you to be able to play the game itself - no.
you dont have to be online, but you wont be able to play with others (if you have gold)
The game has to be able to go online.
Well it depends on what games. Most games u can play xbox live with 2 people. For example on halo 3 u can play 4 people on xbox live...
You don't need a computer as long as you have internet to hook up your Xbox to you will be able to play Xbox Live.
If you did get XBOX Live you are able to play online (the main feature) and get extras for your games of XBOX Live Market Place watch films and listen to music.
You do NOT need Xbox live to play Halo Wars. Xbox live is only used if you want to play with other people and to update the game.
No. Left 4 Dead Xbox Live Players can only play with other Xbox players. The same goes for PC.