Server :: Create New /etc/aliases For Postfix?

I have /var on it's own partition. Should i direct mail, in /etc/alises, sent to root to my normal local user or will /var end up getting filled up with mail, if i do that?Also:I went on the forums for dynDNS.com, explaining that i was setting up a network that includes a web server and a mail server. They rightly pointed out that i only needed to set up a DNS A record for webserver.dyndns.org. The mail server would be located by this DNS A record, as both machines share the same external I.P address(the external I.P of my ADSL router). So my mail addresses could be me@webserver.dyndns.org. So both machines would be reachable with the same DNS A record, and port-forwarding on my ADSL router would determine which host got which requests. That's what they said i should do for my scenario.
Bearing this in mind what should the values of $myhostname, $mydomain, $mydestination and $myorigin be?

My current thoughts, on the simplest way to do this, are:If $myhostname is mailserver.webserver.dyndns.org, then $myorigin can be webserver.dyndns.org, then $mydomain is webserver.dyndns.org and $mydestination has it's DEFAULT setting(mydestination = $myhostname localhost.$mydomain localhost).This the simplest workable approach right?, if not what are the best values for these parameters?.Also I have /var on it's own partition. Should i direct mail, in /etc/alises, sent to root to my normal local user or will /var end up getting filled up with mail, if i do that?