I see this device does Port Forwarding (As it calls it) in a way that is closer to Port Triggering to one IP address.
http://portforward.com/english/routers/port_forwarding/Dlink/DIR-655/iChat.htm
It will probably pay to use Port Triggering (Applications in that menu) as there are two computers.
The PC will need the Trigger to be port 5190 and so will iChat for some stuff.
There the ports to be opened become
Trigger port 5190 on TCP to Open Ports 5190, 1024-5000 TCP and 5190 on UDP
Then the iChat side will need
Port 5678 on UDP to Trigger(open) 16393-16402 on UDP.
If you use Jabber or Bonjour set them up as single ports to a line.
3:43 PM Saturday; December 29, 2007