Wes,
Take a look at 
https://secure.dd-wrt.com/wiki/images/7/7f/DD-WRT_Port_Forward_Web_GUI_v23_SP1.png and I will try to walk you through this.
Let’s assume the following:
Internal Router IP:  192.168.1.1
DNS-321 IP: 192.168.1.20
Printer IP: 192.168.1.30
MP3 Player: 192.168.1.40
External Router IP: 97.96.120.200
You would set your port forwarding similar to this:
Application     Port From     Protocol      IP Address          Port To
Router           8080              TCP           192.168.1.1         80
DNS-321        8081              TCP           192.168.1.20       80
Printer           8082              TCP           192.168.1.30       80
MP3              8083              TCP           192.168.1.40       80
The Application field is just a comment field.  Do not forget to set “Enable”
From outside your network, you would access your router using: 97.63.120.200:8080.  The router would look at port 8080 and know to forward that to port 80 at your internal IP address of 192.168.1.1.
From outside your network, you would access your DNS-321 using: 97.63.120.200:8081.  The router would look at port 8080 and know to forward that to port 80 at your internal IP address of 192.168.1.20.
I would also recommend using dyndns (or any service you prefer) to map your external address to something like wes.dyndns.org, then you could access your DNS-321 with wes.dyndns.org:8081.