It depends if you game supports a same system multiplayer online feature, like Halo 3. If you game does support that feature then you will probably need to sign everyone in except for yourself as a guest profile for xbox live.
System Link
No, the Wii and Xbox do not connect for any game that's not possible. The Wii online gameplay is much slower due to the controls. If Xbox and Wii connected together on most games, (especially Call of Duty) the Xbox players would have unfair advantages and pwn the noob Wii players.
No. PS3 and Xbox has different servers.
No, players playing online on a computer version of a game cannot play with players online on the Xbox 360 version of the game.
No, PS3 players cannot play with Xbox 360 players online.
No
PS3 can only play online on the PlayStation Network and Xbox only on Xbox live so they are on different networks and can not play each other
No
XBOX Live is not its own game system. XBOX Live is an online gaming and content delivery service that the XBOX 360 game system uses.
Xbox players are unable to play online with PlayStation players because they online play is run through two different domains.
They can not. I do not know the reason for this but I know that online games are not mixed with the PS3 console.
If you are playing online there are 16. Xbox 360 live =16 PS3 online =16