How do I display user “display name”?

I can currently display the $user_login (username) of a user on our homepage – eg: “hello username!”, but instead, I would like to display the “Display Name” that the user selects in their profile. – eg: “hello display_name!”

My current code is:

<?php
get_currentuserinfo() ;
global $user_level;
if ($user_level > 0) {
echo ("hello " . $user_login . "!");
} ?>

I would like it to be:

<?php
get_currentuserinfo() ;
global $user_level;
if ($user_level > 0) {
echo ("hello " . $user_display_name . "!");
} ?>

Any ideas?