Hints for a messenger

I`m working for a messenger from a while... and i have some question...

1) how to receive friend list?
It`s a good idea to make a list in a bidimensional array on the server and send it to client?

example:
server side:
string[][] List = new string[nr][];

... populate the list... and send it to client

or... if this isn`t a professional solutin... I need some hints...

2) my messenger it`s a little weird... :-s
When I sing in, the login information are send to server... and the server are getting my IP and return my friend list (and theirs IPs). In every client are implemented a server and a client.
When I want to speak with someone... I can connect directly to his server.
So... after login, the MainServer it`s there just for checking me to see if I`m oline (from time to time)

Again... it`s this a good solutin?
My messenger will work a lot with connexions... and i share movies, images, music and a lot of things... i realy need a high connection.

Thanks