messenger and router firewall

I was wondering has anyone out there tried to run messenger on 2 pcs behind
a d-link firewall. I have the DI-704up which is plug and play compatable
and have my plug and play enabled on windows XP. The problem is I would like
to voice messenger and video while still being able to chat and send files
at the same time with the other machine(also XP). According to the D-link
manaul messenger will tell the router what ports it needs open but I don't
know if it opens all of them at once or just the ones you need.The manual
says you can only open ports to one machine at a time!

Anyhelp would be appreciated.
Ron