On MX, yeah, just one entry - put your priority to 1, then the IP of your server.
When you install roundcube via apt-get, it'll just install the PHP script. You need to/can (depending on how you want to look at it) install your own mail servers beforehand - sendmail or postfix + dovecot or courier.
There's also iRedmail (
http://www.iredmail.org/download.html) which pretty much handles it all -- installing Postfix, Dovecot, PHP, MySQL, Apache, Roundcube.. -- so you don't need to muck with configurations and stuff