I'm not sure about postfix and dovecot since I've never used them (I use Qmail). You might run into some problems with your IP address being blocked by other mail servers since it's likely to be an IP allocated for Internet access and not web/email services. You can check for this on your IP at
http://www.spamhaus.org/zen/index.lasso using the IP search box on the left side. My guess is your IP will be in the PBL.
To get multiple web servers running from a single IP address you would need a proxy server behind your router and in front of your web servers.
You'll need to setup public DNS for your domains and have them pointed to your public IP address (do this step last if your websites are currently active and you want to minimize downtime).
Have your router automatically forward port 80 traffic to your proxy server.
Setup DNS on the proxy server so the IP's returned for all your domains point to the internal IP address of whichever webserver is associated with each domain.
Data flow will be as such:
Internet -> your IP/router -> your proxy server -> web server #1 or #2 (depending on domain)
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